OB 


BOOKS    BY    DR.    FISCHER 

PUBLISHED  BY 

JOHN  WILEY  &  SONS,  Inc. 
432  Fourth  Avenue  New  York 

(Edema  and  Nephritis. 

A  Critical,  Experimental  and  Clinical  Study  of  the  Physiology 
and  Pathology  of  Water  Absorption  in  the  Living  Organism. 
Second  and  enlarged  edition.  695  pages,  6x9,  159  figures. 
Cloth,  $5.00  net  (2 1/-  net). 

The  Physiology  of  Alimentation. 

viii+348  pages,  5J-  x  8,  30  figures.    Cloth,  $2.00  net  (8/6  net) 

Fats  and  Fatty  Degeneration. 

A  Physico-Chemical  Study  of  Emulsions  and  the  Normal  and 
Abnormal  Distribution  of  Fat  in  Protoplasm.   (With  Dr.  Marian 
O.  Hooker.) 
viii+155  pages,  6  x  9,  65  figures.    Cloth,  $2.00  net  (8/6  net). 

TRANSLATIONS 

Physical  Chemistry  in  the  Service  of  Medicine. 

Seven  Addresses  by  Dr.  Wolfgang  Pauli,  Professor  in  the 
Biological  Experiment  Station  in  Vienna.  Authorized  Trans- 
lation by  Dr.  Martin  H.  Fischer. 

ix+156  pages,  5  x  7J.     Cloth,  $1.25  net  (5/6  net). 

An  Introduction  to  Theoretical  and  Applied  Colloid 
Chemistry. 

Five  Lectures  by  Dr.  Wolfgang  Ostwald,  Privatdozent  in  the 
University  of  Leipzig.     Authorized  Translation  by  Dr.  Martin 
H.  Fischer, 
xiv  +  232  pages,  6x9,  45  figures.     Cloth,  $2.50  net. 


; 


Frontispiece 


THOMAS  GRAHAM 


AN  INTRODUCTION  TO . 

THEORETICAL  AND  APPLIED 
COLLOID  CHEMISTRY 

"THE    WORLD    OF   NEGLECTED    DIMENSIONS" 


BY 
DR.   WOLFGANG   OSTWALD 

Privatdozent  in  the  University  of  Leipzig 

AUTHORIZED   TRANSLATION   FROM   THE   GERMAN 
BY 

DR.    MARTIN    H.    FISCHER 

Eichberg  Professor  of  Physiology  in  the  University  of  Cincinnati 


FIRST  EDITION 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:   CHAPMAN   &   HALL,    LIMITED 
1917 


COPYRIGHT,  1917,  BY 
MARTIN  H.  FISCHER 


Stanbope  ipress 

H.GILSON   COMPANY 
BOSTON,  U.S.A. 


TO 

lr.  iHartut  ij 

Professor  of  Physiology  in  the  University  of  Cincinnati 
IN  SINCERE  FRIENDSHIP 


PREFACE. 

THIS  small  volume  is  the  literary  result  of  a  series  of 
lectures  which  I  gave  during  the  winter  of  1913  and  1914  in 
the  United  States  and  Canada  upon  the  invitation  of  a 
number  of  American  universities.  Originally  invited  by 
five  universities,  I  found  the  interest  in  the  science  with 
which  this  volume  deals  so  great  that  their  number  grew  to 
sixteen  while  the  actual  number  of  lectures  demanded  of  me 
during  some  seventy-four  days  was  fifty-six.  Lack  of 
time  and  strength  compelled  me  then  to  forego  the  pleasure 
of  accepting  further  invitations.  By  way  of  expressing  my 
thanks  and  my  appreciation  of  the  friendliness  and  the  high 
honor  of  these  invitations  and  in  order  to  send  greetings 
once  more  to  my  many  scientific  friends  on  the  other  side, 
I  beg  to  list  the  universities  and  institutions  in  which  it  was 
my  privilege  to  discuss  colloid  chemistry.  They  are  the 
University  of  Cincinnati  (where  I  spoke  under  the  auspices 
of  the  Cincinnati  Society  for  Medical  Research  and  the 
Cincinnati  branch  of  the  American  Chemical  Society);  the 
University  of  Illinois;  Columbia  University,  the  College 
of  Physicians  and  Surgeons  and  the  College  of  the  City  of 
New  York  in  New  York  City;  Johns  Hopkins  University 
and  the  Johns  Hopkins  Medical  School  in  Baltimore;  the 
University  of  Chicago;  the  Ohio  State  University;  McGill 
University;  the  Mellon  Institute  of  the  University  of  Pitts- 
burgh; the  University  of  Nebraska;  the  University  of 
Kansas;  before  the  American  Chemical  Society  in  Indian- 
apolis; before  the  National  Academy  and  the  American 
Chemical  Society  in  Washington. 

If  I  have  omitted  any  institution  or  scientific  body  to 
which  I  had  the  pleasure  of  addressing  myself  and  which 
in  consequence  did  its  share  toward  making  possible  the 
lectures  given  in  this  volume,  I  ask  pardon.  I  admit  that 
I  had  difficulty  in  remembering  everything  that  happened 

ix 


X  PREFACE 

to  me  while  living  at  what  seems  to  be  the  customary 
American  rate.  I  need  to  express  my  appreciation,  also, 
of  invitations  received  from  the  Massachusetts  Institute 
of  Technology,  the  University  of  California,  Syracuse  Uni- 
versity and  a  number  of  others  —  invitations  which  I  regret 
it  was  impossible  to  accept. 

It  hardly  needs  to  be  emphasized  that  upon  such  a  tour 
the  lecturer  learns  quite  as  much  as  his  audience.  The 
necessity  of  having  his  material  so  easily  in  hand  that  he 
may  vary  it  according  to  the  type  and  special  wishes  of  his 
audience,  or  according  to  the  time  at  his  disposal,  or  to  suit 
the  viewpoint  from  which  it  is  expected  that  his  subject 
shall  be  handled  —  these  things  are  of  the  greatest  value  to 
the  lecturer  himself.  There  is  obviously  much  difference 
between  the  half-popular  dissertation  on  colloid  chemistry 
which  is  given  twelve  or  thirteen  hundred  freshmen  fore- 
gathered in  a  building  ordinarily  used  for  religious  exercises 
and  the  talk  which  is  given  so  select  an  audience  as  the 
American  National  Academy  and  the  American  Chemical 
Society  meeting  in  the  spacious  halls  of  the  Cosmos  Club. 
And  the  theme  of  colloid  chemistry  is  itself  made  to  wear  a 
different  face,  depending  upon  whether  one  talks  the  week 
through  in  Pittsburgh  to  workers  interested  chiefly  in 
technical  problems  or  whether  one  tries  hi  two  hours  in  the 
Johns  Hopkins  Medical  School  in  Baltimore  to  discuss  the 
relationships  of  colloid  chemistry  to  biology  and  medicine. 

Besides  such  possibilities  for  arranging  and  rearranging 
his  materials,  other  advantages  accrue  to  the  lecturer.  He 
is  hi  this  way  enabled  to  determine  by  experiment,  as  it 
were,  what  is  the  best  form  and  the  most  easily  intelligible 
one  in  which  he  can  present  his  remarks,  and  what  are  the 
facts  and  thoughts  which  interest  his  audience  most.  He 
needs  but  to  observe  how  it  reacts  to  his  mode  of  presenta- 
tion. He  soon  discovers  what  of  that  which  he  presents  is 
not  clear  to  his  audience,  is  superfluous,  or  unduly  long; 
what,  on  the  other  hand,  interests  them  most;  and  in  the 
discussions  which  follow  a  lecture  he  soon  discovers  whether 


PREFACE  xi 

he  has  succeeded  in  making  his  main  argument  clear. 
These  things  are  possible,  of  course,  only  when  the  psycho- 
logical experiment  can  be  made  many  times.  How  fruitful 
may  be  such  an  experiment  tried  in  succession  upon  a  series 
of  new  audiences  is  best  evidenced,  perhaps,  by  the  fact  that 
in  the  course  of  these  lectures  both  the  choice  of  material 
and  its  disposition  in  the  various  lectures  underwent  a  steady 
change.  It  may  fairly  be  said  that  what  has  been  chosen 
for  presentation  in  this  volume  is  the  product  of  this  experi- 
ence. This,  and  the  generous  request  of  American  friends 
that  1  print  them,  has  led  me  to  select  the  five  lectures  which 
I  gave  most  often,  to  dictate  them  and  to  bring  them  out 
in  this  form. 

There  already  exist  a  number  of  strictly  scientific  text- 
books treating  of  colloid  chemistry  and  a  number  of  more  or 
less  valuable  introductions  to  colloid  chemistry  of  a  popular 
or  semi-popular  nature.  So  far  as  I  know,  however,  none 
of  these  has  tried  to  establish  the  right  of  modern  colloid  chemistry 
to  existence  as  a  separate  and  independent  science  while  em- 
phasizing at  the  same  time  its  great  possibilities  of  scientific 
and  technological  application.  The  attempt  to' give  a  general 
survey  of  modern  colloid  chemistry  as  a  pure  and  as  an  applied 
science  and  in  a  form  readily  intelligible  to  the  general  reader 
seems  to  be  new. 

This  volume  makes  its  first  appeal  to  such  readers  as  have 
heard  little  or  nothing  of  colloid  chemistry.  It  was  to 
several  thousand  of  just  such  that  I  gave  these  lectures,  and 
it  was  through  frequent  contact  with  them  that  I  was  led, 
time  after  time,  to  change  my  mode  of  presentation,  and, 
I  hope,  to  improve  it.  I  had,  however,  another  reason 
for  thus  addressing  myself  to  such  readers.  There  still 
exists,  I  think,  too  great  an  hiatus  between  the  true  signifi- 
cance, importance  and  application  possibilities  of  modern 
colloid  chemistry  and  the  knowledge  which  the  public  has 
of  this  science.  It  is  a  fair  statement  that  every  scien- 
tifically cultured  individual  knows  something  about  radio- 
chemistry.  But  that  with  radio-chemistry  there  was  born 


Xll  PREFACE 

a  twin  science,  the  fruits  of  which  are  no  less  wonderful  and 
the  application  possibilities  of  which  to  all  possible  branches 
of  science,  to  technology  and  to  industry  are  not  only  equal 
to  but  exceed  those  of  radio-chemistry  —  this  seems  still 
largely  unknown  to  the  general  public.  I  do  not  hesitate 
in  consequence  to  designate  this  volume  a  propaganda  sheet 
for  colloid  chemistry. 

I  am  also  presumptuous  enough  to  believe  that  I  shall, 
through  this  book,  be  able  to  render  some  of  my  colleagues 
in  colloid  chemistry  a  small  service.  It  is  a  cause  for  re- 
joicing that  the  colloid  chemist  is  being  asked  more  and 
more  frequently  to  address  audiences  upon  the  general 
fruits  of  modern  colloid  chemistry.  These  lectures  may, 
perhaps,  render  him  aid  in  such  circumstances.  I  would 
especially  emphasize  the  rather  lengthy  footnotes  in  which 
experiments  are  frequently  discussed  which  have  the  great 
merit  of  always  "  going."  I  have  also  written  into  this 
volume  a  number  of  not  previously  published  opinions  and 
experiments  which  the  expert  worker  in  colloid  chemistry 
will  readily  discover  for  himself;  and  in  the  footnotes  I 
have  often  tried  to  give  expression  to  suggestions  which 
come  into  one's  mind,  one  might  almost  say  automatically, 
whenever  one  works  long  and  hard  in  a  given  field.  But 
the  professional  colloid  chemist  will,  perhaps,  be  most 
interested  in  just  what  concepts  and  facts  I  chose  for 
presentation,  because  they  seemed  to  me  to  be  especially 
characteristic  of  modern  colloid  chemistry. 

Because  of  the  wealth  of  colloid-chemical  papers  and 
books,  I  could  not  hope  to  give  references  to  more  than  a 
few.  In  choosing  those  which  I  did,  I  have  selected  for  the 
most  part  such  papers  and  larger  texts  as  contain  summaries 
of  investigations  and  are  ready  guides  to  further  literature. 

May  this  volume  serve  my  readers  as  a  guide  into  a  long- 
existent  but,  until  recently,  scarcely  recognized  world  of 
remarkable  phenomena  and  wondrous  mental  concepts. 

WOLFGANG  OSTWALD. 

GROSSBOTHEN,  WALDHAUS 
July,  1914. 


TABLE   OF   CONTENTS. 

PAGE 

FIRST  LECTURE 3 

Fundamental  Properties  of  the  Colloid  State.  Colloids  as  Ex- 
amples of  Dispersed  Systems.  Methods  of  Preparing  Colloid 
Solutions. 

SECOND  LECTURE 39 

Classification  of  the  Colloids.  The  Physico-Chcmical  Properties 
of  the  Colloids  and  their  Dependence  upon  the  Degree  of  Dispersion. 

THIRD  LECTURE 79 

The  Changes  in  State  of  Colloids. 

FOURTH  LECTURE 127 

Some  Scientific  Applications  of  Colloid  Chemistry. 

FIFTH  LECTURE 179 

Some  Technical  Applications  of  Colloid  Chemistry. 

INDEX.  .  223 


xm 


I. 

FUNDAMENTAL  PROPERTIES  OF  THE  COLLOID 

STATE. 

COLLOIDS  AS  EXAMPLES  OF  DISPERSED  SYSTEMS. 
METHODS  OF  PREPARING  COLLOID  SOLUTIONS. 


COLLOID  CHEMISTRY 


FIRST  LECTURE. 

FUNDAMENTAL  PROPERTIES  OF  THE  COLLOID 
STATE.     COLLOIDS    AS    EXAMPLES    OF    DIS- 
PERSED   SYSTEMS.     METHODS    OF    PRE- 
PARING COLLOID  SOLUTIONS. 

I  HAVE  had  the  honor  of  being  asked  to  tell  you  something 
of  a  new  branch  of  physical  chemistry,  namely,  colloid 
chemistry.  Although  I  know  full  well  that  one  should 
never  begin  a  lecture  with  an  apology,  I  feel  that  a  few 
remarks  are  necessary  before  I  enter  upon  my  main  theme. 

As  you  know,  colloid  chemistry  is  a  relatively  young 
science.  Colloid  chemistry  was  officially  founded  by  the 
Englishman,  THOMAS  GRAHAM,  some  fifty  years  ago. 
Various  papers,  it  is  true,  were  written  even  earlier  on  sub- 
jects which  we  today  regard  as  colloid-chemical.  I  need 
but  to  call  to  your  mind  the  contributions  of  the  German, 
BENJ.  JEREMIAS  RICHTER,  and  of  the  Italian,  F.  SELMI, 
which  appeared  in  the  beginning  of  the  nineteenth  century. 
Yet  it  is  only  in  the  last  ten  years  that  the  facts  of  colloid 
chemistry  have  become  sufficiently  large  in  number,  that 
their  relationships  to  each  other  have  become  sufficiently 
plain,  and  that  laws  regarding  their  behavior  have  been 
discovered  which  justify  a  discussion  of  colloid  chemistry 
as  a  separate  branch  of  science. 

But,  though  still  so  young,  the  phenomena  and  the  ideas 
incorporated  under  the  term  colloid  chemistry  are  many. 
It  is  an  almost  universal  complaint  that  colloid  chemistry 
has  already,  —  I  beg  you  to  note,  already,  —  become  so 
great  that  no  one  man  can  master  it  in  its  entirety.  Espe- 

3 


4  COLLOID  CHEMISTRY 

cially  does  the  beginner  find  it  difficult  to  get  about  in  the 
confusing  mass  of  colloid-chemical  facts  and  theories.  Nor 
has  this  rapid  development  in  any  sense  ceased  or  even 
abated.  The  reverse  is  the  case.  New  facts  and  new  ideas 
are  born  daily  as  they  were  ten  years  ago,  and  a  couple  of 
colloid-chemical  journals  and  some  dozen  text-books  appear 
as  the  mere  beginnings  of  attempts  to  organize  and  classify 
the  available  riches.  No  one  knows  this  better  than  the 
authors  and  founders  of  these  text-books  and  journals. 

This  is  the  point  which  I  should  like  to  emphasize  before 
attacking  my  actual  problem.  It  is  impossible  in  my  short 
series  of  lectures  to  give  more  than  an  outline  of  colloid 
chemistry.  One  can  lecture  for  two  semesters  on  this 
subject  without  doing  it  justice.  It  is  far  easier  to  give 
many  lectures  on  colloid  chemistry  than  only  a  few.  What 
I  bring  you  must  represent  a  mere  instantaneous  photograph 
of  what  I  regard  as  modern  colloid  chemistry.  This  fact 
may  disappoint  those  specialists  among  you  who  would  like 
detailed  discussion  of  some  of  its  special  problems.  I  have, 
however,  been  told  that  what  most  of  my  audience  desires 
is  a  survey  of  the  field,  so  that  what  I  say  represents  an 
effort  to  meet  this  wish. 

II. 

The  first  question  raised  by  any  one  approaching  the  field 
of  colloid  chemistry  is  this:  what  are  colloids?  About  the 
same  feeling  is  expressed  in  the  questions:  what  are  the 
most  important  characteristics  of  a  colloid?  Or,  how  can 
one  determine  quickly  and  simply  whether  or  not  a  given 
substance  is  a  colloid?  This  first  lecture  will  attempt  a  clear 
and  concise  answer  to  these  fundamental  questions.  To 
some  of  you  it  may  appear  that  a  whole  lecture  is  too  much  to 
devote  to  these  questions.  I  could,  of  course,  be  more  brief 
were  I  simply  to  make  two  or  three  statements  defining  the 
concept  colloid  according  to  our  present-day  beliefs  and 
were  I  then  deductively  to  analyze  and  explain  them.  But 
such  a  deductive  method  of  reasoning  would,  to  me  at  least, 


FUNDAMENTAL  PROPERTIES  5 

seem  stupid.  I  believe  that  you  will  like  it  better  if  I  try  to 
present  the  development  of  our  concept  in  a  more  inductive 
and  experimental  manner.  The  answer  to  the  question  is 
by  no  means  as  simple  and  elementary  as  might  at  first 
appear.  It  has  changed  markedly  with  time  and  is  today 
quite  different  from  what  it  was  in  GRAHAM'S  day  or  in  the 
older  text-books.  How  important  it  is  to  obtain  a  clear  and 
short  definition  of  the  term  colloid  is  clearly  betrayed  by 
the  fact  that  there  is  often  no  end  to  debates  in  colloid 
chemistry  because  different  authors  have  each  different 
notions  of  what  really  constitutes  a  colloid. 

One  might,  at  first  sight,  think  it  possible  to  decide  whether 
a  given  substance  is  a  colloid  or  not  on  the  basis  of  its 
general  chemical  or  physical  properties.  The  word  colloid 
is  derived  from  the  Greek  KO\\<X  meaning  glue.  Thus  one 
might  think  that  all  chemically  complex  substances  are 
colloids.  This  conclusion  is  partly  justified;  it  is  not,  how- 
ever, universally  true.  More  important  yet  is  the  fact  that 
while  complexity  of  chemical  constitution  is  likely  to  give  rise 
to  the  -colloid  state,  the  converse  does  not  follow.  Among 
the  many  colloids  I  show  you  here,1  you  observe  a  whole 
series  of  very  simple  chemical  composition  (demonstration). 
We  have  here  colloid  sulphids  of  the  heavy  metals,  and  here 

1  I  had  at  my  disposal  many  dry  colloids  prepared  by  PAAL'S  method 
with  the  aid  of  protective  colloids.  Simple  solution  of  a  few  granules  of 
these  (when  necessary  with  the  application  of  heat)  yields  beautiful  and 
lasting  demonstration  solutions.  PAAL  preparations  of  the  various  metals, 
metalloids,  of  mercury  chromate  and  of  manganese  dioxid  are  easily  pur- 
chasable. It  is  also  easy  to  get  colloid  preparations  of  iron  hydroxid  (under 
the  name  of  dialyzed  iron  oxid)  and  colloid  carbon  dispersed  in  an  aqueous 
dispersion  medium  (india  ink,  aquadag)  or  in  a  mineral  oil  (ACHESON'S  oildag). 
One  can  also  buy  colloid  dyes  like  congo  red,  benzopurpurin,  night-blue  and 
alkali-blue.  It  is  an  easy  matter,  also,  to  prepare  colloid  sulphids  of  the 
various  metals  by  working  with  very  dilute  solutions  containing  a  trace  of 
gelatin.  The  same  is  true  of  colloid  berlin  blue,  of  silver  iodid  (KI  +  AgN03) 
and  of  silicic  acid  (NaSiO3  +  HC1).  Examples  of  the  so-called  hydrated 
emulsoids  (like  gelatin)  are  also  easily  obtainable.  These  may  be  dissolved 
in  cold  water  or  when  necessary,  in  hot.  In  the  group  with  gelatin  belong 
agar,  starch,  gum  arabic,  serum  albumin,  casein  (dissolved  in  dilute  alkali) 
and  rubber.  Collodion,  viscose,  etc.,  are  also  easily  obtainable.  The  prep- 
aration of  red  and  blue  colloid  gold  is  discussed  in  the  main  text. 


6  COLLOID  CHEMISTRY 

a  whole  series  of  colloid  elements  like  gold,  silver,  sulphur 
and  carbon.  As  of  especial  enterest  I  show  you  colloid 
sodium  chlorid,  both  as  a  milky  liquid,  and  as  a  jelly 
of  remarkably  beautiful  color.1  No  one,  of  course,  will 
attribute  to  sodium  chlorid  a  complex  chemical  constitu- 
tion, and  yet  you  see  it  here  in  colloid  form.  There  exists, 
therefore,  no  definite  connection  between  chemical  constitu- 
tion and  colloid  state.  In  colloid  chemistry  things  are, 
therefore,  different  from  those  obtaining,  for  instance,  in 
radiochemistry,  where,  as  you  know,  the  observed  phe- 
nomena are  largely  limited  to  elements  of  high  atomic 
weight. 

It  is  also  impossible  to  make  a  list  from  which  one  might 
then  discover  whether  or  not  a  given  substance  is  a  colloid; 
attempts  at  this  were  made  years  ago,  but  proved  unsatis- 
factory from  the  start.  Why  is  such  the  case?  It  is  be- 
cause we  already  know  too  many  colloids  to  make  such 
cataloguing  possible.  While  the  preparation  of  a  single  new 
colloid  was  formerly  regarded  as  of  great  interest,  we  are 
today  familiar  with  methods  by  which  we  can  at  will  convert 
whole  classes  of  substances  into  colloids.  There  is  little 
doubt  that  we  have  often  worked  with  colloids  and  still  do 
so  without  being  aware  of  it.  This  is  true,  for  example,  of 
many  of  the  dyes  and  of  other  organic  substances  of  com- 

1  This  milky  colloid  sodium  chlorid  was  prepared  by  C.  PAAL'S  method 
(see  THE  SVEDBERG,  Herstellung  kolloider  Losungen,  346,  Dresden,  1909). 
A  simpler  and  quicker  method  is  that  of  L.  KARCZAG,  Biochem.  Zeitschr. 
66,  117  (1913)  which  yields  a  particularly  beautiful  jelly-like  colloid.  Judg- 
ing by  my  own  experience,  it  is  best  to  use  thionyl  chlorid  and  sodium  salic- 
ylate,  which  on  double  decomposition  yield  sodium  chlorid  and  a  complicated 
volatile  thionyl  ester.  The  dry  sodium  silicate  is  simply  added  to  a  few  cubic 
centimeters  of  thionyl  chlorid  in  a  test  tube,  the  sodium  silicate  being  allowed 
to  dissolve  in  the  liquid  through  application  of  gentle  heat.  When  about 
0.5  gram  of  the  salt  is  added  to  5  cc.  of  the  liquid,  there  results,  after  cooling 
for  an  hour  or  two,  a  beautiful  glass-like  solid  jelly  exhibiting  in  striking 
fashion  the  so-called  CHRISTIANSEN  refraction  colors  (for  example,  green 
by  reflected  light,  red  by  transmitted  light,  etc.).  These  gels  will  keep  in 
a  closed  tube  for  some  weeks.  If  benzol,  ligroin  or  some  other  substance 
is  used  as  a  diluent,  the  colloid  tends  to  go  to  pieces  more  quickly  than  if 
this  is  not  done. 


FUNDAMENTAL  PROPERTIES  7 

plicated  chemical  structure.  As  we  proceed  we  shall 
encounter  additional  reasons  indicating  why  such  a  listing 
of  colloids  is  impossible. 

We  might  make  attempts  on  other  grounds  to  get  an 
answer  to  our  fundamental  questions,  but  they  would  all 
prove  unsatisfactory.  From  mere  consideration  of  the 
chemical  or  physical  properties  of  a  substance,  we  simply 
cannot  decide  whether  or  not  it  is  a  colloid.  To  make  such 
decision  we  need  to  study  the  properties  exhibited  by  colloid 
substances  under  experimental  conditions  —  we  need  to  make 
a  short ,  qualitative,  colloid-chemical  analysis.  But  when  we 
do  this  we  shall  also  get  an  answer  to  the  question:  how  do 
we  recognize  a  colloid?  And  by  inductive  and  experimental 
methods  we  shall  also  get  answers  to  the  other  questions: 
what  are  the  important  properties  of  a  colloid,  and,  what 
are  colloids  anyway? 

§2. 

It  is  of  interest  that  in  making  such  a  colloid-chemical 
analysis  we  use  experiments  which,  in  a  certain  sense,  follow 
the  historical  development  of  the  whole  subject.  The 
concept  colloid  was  born  of  experiments  on  diffusion.  With 
the  fundamental  phenomenon  of  diffusion  all  of  you  are 
familiar.  If  the  lower  half  of  a  cylinder  is  filled  with  the 
solution  of  a  colored  salt  and  pure  water  is  then  carefully 
poured  upon  it  so  that  admixture  does  not  occur,  the  salt 
slowly  wanders  upwards  into  the  pure  water  even  when  all 
vibration,  etc.,  is  shut  out. 

THOMAS  GRAHAM  was  among  the  first  to  experiment 
extensively  in  this  field  and  to  follow  his  results  quanti- 
tatively. An  important  element  in  GRAHAM'S  work  lay  in 
the  fact  that  he  studied  many  different  kinds  of  substances. 
He  noted  great  quantitative  differences  in  their  diffusibility. 
While  some  dissolved  substances,  such  as  salts,  acids  and 
bases,  showed  a  considerable  diffusion  velocity,  he  noted 
little  or  no  diffusion  in  the  case  of  gelatin,  albumin,  silicic 
acid  and  aluminium  hydroxid.  Those  which  diffused  but 


8 


COLLOID   CHEMISTRY 


little  or  not  at  all  GRAHAM  called  colloids.  This  simple 
observation  constitutes  the  foundation  of  the  science  of 
colloid  chemistry. 

It  is  easily  seen  that  such  diffusion  experiments  are  hard 
to  carry  out  quantitatively  when  the  pure  solvent  is  simply 
laid  over  a  solution,  for  slight  variations  in  technic  and 

currents  due  to  temperature  differ- 
ences disturb  their  accuracy.  It 
is  well-nigh  impossible  to  demon- 
strate such  experiments  on  "free" 
diffusion  to  a  large  audience.  It 
is,  however,  possible  to  carry  out 
these  experiments  in  a  more  stabile 
form,  the  basic  principle  of  which 
was  also  recognized  by  GRAHAM. 
It  rests  upon  the  fact  that  the 
velocity  of  diffusion  in  dilute  gels 
-  but  only  in  dilute  ones  —  is 
practically  the  same  as  in  the 
pure  solvent.  I  show  you  here 
several  test  tubes  half  filled  with 
three  per  cent  gelatin  (demonstra- 
tion). Upon  them  were  poured  a 
series  of  colored  solutions  which 
were  then  permitted  to  diffuse 
down  into  the  gelatin  for  some 
days.  You  observe  that  the  blue 
copper  sulphate  and  the  yellow 
picric  acid  have  penetrated  deeply 
into  the  gela.tin.  On  the  other 
FIG.  l.— Diffusion  of  a  col-  hand,  the  tubes  into  which  colloid 
loid  (d)  and  a  "  true "  solution  gold,  silver,  iron  hydroxid  and 

(6)  into  solid  gelatin.  CQngo  red  were  poured  ghow  nttle 

diffusion.  The  series  demonstrates  clearly  the  different 
degrees  of  diffusibility  of  the  different  substances,  just  as 
GRAHAM  first  observed.  Let  me  emphasize  at  once  that  as 
shown  in  Fig.  1  these  simple  diffusion  experiments  prove  of 


FUNDAMENTAL  PROPERTIES 


9 


great  service  in  any  attempt  to  determine  the  colloid  (a)  or 
non-colloid  (6)  character  of  a  given  solution. 

There  is  another  way  of  overcoming  the  difficulties  inci- 
dent to  experiments  on  free  diffusion  which  permits,  perhaps, 
a  still  sharper  distinction  between  diffusing  and  non-diffusing 
substances.  Presumably  in  an  effort  to  overcome  the  ex- 
perimental difficulties  incident  to  his 
systematic  study  of  many  solutions 
GRAHAM  resorted  to  the  following:  Sup- 
pose we  imagine  a  membrane  of  some 
sort,  capable  of  taking  up  the  solvent,  to 
be  introduced  between  a  salt  solution 
and  its  pure  solvent.  If  a  parchment 
paper  tube,  such  as  I  show  you  here,  is 
filled  with  a  solution  and  the  whole  is 
then  placed  in  a  beaker  filled  with  the 
pure  solvent  as  shown  in  Fig.  2,  it  is 
clear  that  diffusion  may  occur  through 
the  parchment  paper  without  being  sub- 
ject to  disturbances  due  to  vibration, 
etc.,  at  the  surface  of  contact  between 
solution  and  solvent.  Diffusion  experi-  FIG.  2.  —  Dialyzer  ar- 
ments  of  this  type  were  also  first  made  ransed  for  colloid  analy- 

oiq 

by  GRAHAM;  in  fact,  he  gave  them  a 
special  name,  calling  the  diffusion  of  dissolved  substances 
through  membranes,  dialysis.  He  found  that  the  substances 
capable  of  free  diffusion  passed  through  these  membranes, 
while  those  incapable  of  diffusion  did  not.  A  distinction  of 
non-colloid  from  colloid  solutions  could,  therefore,  be  made 
by  dialytic  means. 

Parchment  paper  is,  of  course,  not  the  only  substance  from 
which  such  membranes  may  be  prepared.  Pig  bladder, 
fish  bladder,  sausage  casings,  reed  tubes,  or  collodion  may 
be  used.  They  may  be  used  in  connection  with  and  to 
cover  special  pieces  of  apparatus,  such  as  bells  or  rings.  I 
show  you  here  a  number  of  such  dialyzers,  directing  your 
especial  attention  to  two,  constructed  according  to  GRAHAM. 


10  COLLOID  CHEMISTRY 

To  prepare  artificially  a  dialytic  membrane  as  one  of  col- 
lodion, filter-paper  capsules  need  simply  be  soaked  in  a 
collodion  solution  after  which  the  solvent  for  the  collodion 
is  allowed  to  evaporate.1 

As  you  see,  we  have  already  uncovered  experimentally 
two  characteristics  of  colloids.  I  imagine  now  that  I  hear 
you  say:  that  is  all  very  simple;  colloid  solutions  are 
solutions  which  do  not  diffuse  and  which  do  not  dialyze. 
This  would  constitute  an  experimental  definition  of  colloid 
solutions.  No  doubt  we  have  discussed  two  of  the  most 
important  experimental  characteristics  of  colloid  solutions, 
but  when  we  look  at  the  problem  more  closely  we  discover 
that  these  do  not  suffice  to  characterize  them  fully.  More- 
over, a  little  thought  reveals  that  this  definition  rests  upon 
certain  theoretical  assumptions  which  may  not  be  taken  for 
granted.  Let  us  take  up  this  point  for  a  moment. 

§3. 

Trouble  arises  from  use  of  the  word  solution  in  our  defini- 
tion. What  do  we  mean  by  this  term?  Let  us  for  the 
moment  free  our  minds  of  all  special  hypotheses  regarding 
its  nature.  What  we  regard  as  characteristic  of  a  solution 
is  that  it  represents  a  molecular  distribution  of  one  substance 
in  a  second.  Is  this  requisite  fulfilled  in  the  case  of  colloid 

1  Following  the  method  of  G.  MALFITANO,  collodion  capsules  are  best 
prepared  by  dipping  clean  test  tubes  into  liquid  collodion.  To  get  the 
collodion  evenly  distributed,  the  tubes  are  turned  in  the  air  while  evapora- 
tion of  the  solvent  is  taking  place.  After  being  thus  dried,  the  collodion 
films  are  stripped  from  the  tubes.  Dialysis  thimbles  may  also  be  prepared 
by  coating  the  inside  of  Erlenmeyer  flasks  with  collodion  and  pouring  off 
any  excess.  Everyone  who  has  worked  with  these  collodion  sacks  knows 
that  their  preparation  is  associated  with  a  whole  series  of  small  technical 
tricks.  The  parchment  thimbles  of  Schleicher  and  Schull  are  convenient  for 
many  colloid-chemical  purposes  but  are  rather  costly.  They  often  have 
an  acid  reaction  and  should  therefore  be  washed  in  boiling  water  before  use. 
The  saturation  of  filter-paper  thimbles  with  collodion,  as  described  in  the 
text,  is  much  simpler  and  cheaper,  since  filter-paper  thimbles  can  be  obtained 
in  all  sizes  and  are  relatively  cheap.  What  is  most  important,  however,  is 
that  diffusion  capsules  thus  prepared  are  very  strong.  They  may  also  be 
used  as  osmometers. 


FUNDAMENTAL  PROPERTIES  11 

solutions?  Are  " molecules"  floating  about  in  them?  The 
older  authors,  including  GRAHAM,  believed  this  to  be  true 
even  though  they  did,  of  course,  think  that  there  was  some 
sort  of  a  difference  between  the  molecules  of  a  colloid  and 
those  of  a  non-colloid.  A  first  attempt  to  define  this  con- 
sisted in  pointing  out  a  possible  physical  difference  between 
the  molecules  of  the  two,  as  illustrated,  for  instance,  hi  the 
phenomena  of  allotropism.  Sulphur,  for  example,  hi  its 
different  allotropic  forms  possesses  different  physical  char- 
acteristics. There  is,  for  example,  rhombic  and  hexagonal 
sulphur  and  sulphur  as  STJ  and  SX.  In  some  such  ill-defined 
manner  GRAHAM  and  his  followers  accounted  for  the  differ- 
ences between  the  molecules  of  a  colloid  and  a  non-colloid 
solution.  CAREY  LEA,  one  of  the  best  known  of  American 
colloid  chemists,  gave  his  paper  on  the  colloid  solutions  of 
metals  the  title,  " Allotropic  Modifications  of  Silver" 
actually  meaning  new  colloid  forms  of  it. 

Suppose,  for  the  moment,  that  we  assume  this  view  to  be 
correct  and  that  we  actually  do  deal  both  in  solutions  of 
colloids  and  of  non-colloids  with  molecules  of  the  orthodox 
type  but  possessed  of  different  physical  properties.  There 
really  do  exist  many  similarities  between  colloid  and  ordi- 
nary molecular  solutions.  Thus  many  colloid  solutions  like 
those  of  red  gold,  of  congo'red  or  of  berlin  blue  are  just  as 
clear  to  the  naked  eye  as  molecular  solutions  of  fuchsin  or 
copper  sulphate  (demonstration).  But  colloid  solutions 
also  behave  like  ordinary  molecular  solutions  in  that  they 
pass  unchanged  through  paper  filters  and  even  through  most  of 
the  very  fine  porcelain  or  clay  filters.  I  can  prove  this  to  you 
with  any  of  the  colloids  here  on  the  table,  as  with  this  colloid 
gold  or  colloid  indigo  (demonstration).  These  phenomena 
emphasize  the  great  similarities  and  close  relationships 
between  colloid  and  ordinary  molecular  solutions. 

Let  me  show  you  an  experiment  which  will  recall  your 
first  days  in  qualitative  analytical  chemistry.  I  have  here 
a  saturated  solution  of  mercuric  cyanid  to  which  I  add  some 
hydrogen  sulphid.  You  see  that  mercuric  sulphid  is  pro- 


12  COLLOID  CHEMISTRY 

duced  (demonstration).  A  thick  precipitate  is  formed 
which  quickly  settles  and  which  we  can  then  readily  filter 
off  (demonstration).  Only  a  practically  colorless  solution 
passes  through  the  filter.  Let  me  now  repeat  the  experi- 
ment, but  this  time  I  shall  use  a  very  dilute  cyanid  solution 
(demonstration).  As  you  see,  mercuric  sulphid  is  again 
produced  which  must  this  time  also  be  solid  for  it  is  insoluble 
in.  water  or  in  a  dilute  solution  of  hydrocyanic  acid.  I  again 
pour  the  dark  brown  solution  upon  a  filter,  but  you  observe, 
we  encounter  what  is  so  unpleasant  to  the  analyst:  the 
precipitate  goes  through  (demonstration).  What  are  we 
to  do?  Is  the  " notoriously"  insoluble  sulphid  of  mercury 
prepared  from  the  dilute  solution  also  a  colloid?  We  know 
that  it  is  a  precipitate,  and  a  precipitate  of  a  solid  substance, 
for  mercuric  sulphid  at  room  temperature  and  in  the  pres- 
ence of  water  can  be  nothing  else.  If  the  concentration  is 
merely  raised,  or  the  solution  is  left  to  itself  for  a  time,  or 
if  we  add  salt,  we  obtain  a  solid  precipitate  from  this  brown 
liquid,  as  every  analyst  knows,  yet  this  brown  liquid  which 
has  passed  through  the  filter  and  which  contains  the  sulphid 
precipitate  looks  just  as  clear  to  the  naked  eye  as  any 
ordinary  filtered  molecular  solution.  We  can  also  carry  out 
diffusion  and  dialysis  experiments  with  this  finely-divided 
precipitate,  for  it  will  keep  for  days. 

In  the  series  of  tubes  illustrating  diffusion  there  is  one 
filled  with  just  such  a  mercuric  sulphid  precipitate  as  we  are 
discussing  and  you  notice  that  none  of  it  has  wandered  down 
into  the  gelatin.  The  precipitate  therefore  behaves  like  a 
colloid  in  this  regard  also.  But  may  we  under  the  circum- 
stances still  continue  to  speak  of  molecular  division?  Are 
all  the  other  colloids  we  have  before  us  nothing  more  than 
such  finely-divided  precipitates  —  the  idea  seems  rather 
plausible  —  are  they  nothing  but  very  fine  suspensions  of 
insoluble  substances,  nothing  but  "mechanical"  suspensions 
or  emulsions  produced  by  mixing  an  insoluble  solid  or  liquid 
into  a  second  liquid  menstruum  ?  No  doubt  these  facts  will 
convince  you  that  inability  to  diffuse  and  to  dialyze  are 


FUNDAMENTAL  PROPERTIES  13 

alone  not  sufficient  to  characterize  colloid  solutions.  Mere 
suspensions  of  finely-divided  precipitates  also  do  not  diffuse 
or  dialyze. 

§4- 

Connected  with  these  peculiar  relations  of  the  colloids  to 
the  ordinary  solutions  and  of  the  colloids  to  the  mechanical 
suspensions  there  is  a  most  interesting  and  vital  debate. 
On  the  one  hand,  investigators  have  tried  to  make  colloid 
solutions  simply  a  subdivision  of  molecular  solutions  and 
something  different  from  "mechanical"  suspensions;  on  the 
other  hand,  another  group  has  emphasized  the  similarities 
between  mechanical  suspensions  and  colloids  and  placed 
these  two  together  over  and  against  the  molecular  solutions. 
Their  views  may  be  indicated  as  follows: 

Mechanical  suspensions  Colloids  Molecular  solutions 


They  have  all  tried,  in  other  words,  to  group  the  three 
types  of  substances  under  two  headings.  The  discussion 
has  at  various  times  leaned  now  to  one  side,  now  to  the  other, 
as  the  one  or  other  partisan  believed  he  had  at  last  dis- 
covered a  conclusive  difference  between  the  two  classes. 
Thus,  suspensions  of  the  coarser  precipitates  are  rather 
turbid,  while  many  colloids  are  clear  to  the  naked  eye.  But 
even  FARADAY  learned  to  use  a  special  method  of  illumina- 
tion which  permits  the  recognition  of  slight  turbidities,  and 
so  was  able  to  show  that  red  colloid  gold  is  also  turbid. 
Those  who  grouped  colloid  solutions  with  the  suspensions 
at  once  used  this  fact  as  evidence  for  the  correctness  of  their 
view.  Later,  however,  it  was  found  that  carefully  purified 
concentrated  sugar  solutions  also  appear  turbid  when  the 
FARADAY  method  is  used  upon  them.  On  the  other  hand, 
those  who  grouped  the  colloids  with  the  true  solutions 
pointed  out  that  in  typical,  filterable  colloids  one  can  no 
longer  make  out  the  individual  particles  under  the  micro- 
scope, and  they  used  this  in  support  of  their  view.  It  was 
a  believer  in  this  view,  R.  ZSIGMONDY,  who  was  able  by 


14  COLLOID  CHEMISTRY     , 

optical  methods  to  demonstrate  the  presence  of  individual 
particles  in  typical  colloids,  and  so  again  to  prove  the 
inadequacy  of  this  classification. 

The  discussion  has  not  yet  been  settled.  But  most 
interesting  is  the  fact  that  not  a  single  colloid  chemist  any 
longer  troubles  about  it.  The  discussion  has  simply  dis- 
appeared. And  why?  Because  modern  colloid  chemistry 
teaches  that  there  are  no  sharp  differences  between  mechanical 
suspensions,  colloid  solutions  and  molecular  solutions.  There 
is  gradual  transition  from  the  first  through  the  second  to  the  third. 
It  is  best  to  regard  all  three  from  the  same  viewpoint  and  first 
to  emphasize  their  similarities.  After  this  has  been  done,  their 
special  peculiarities  may  be  taken  up. 

This  constitutes,  perhaps,  the  most  important  conclusion 
of  our  whole  modern  colloid  chemistry.  In  this  lecture  I 
can  only  ask  you  to  take  my  word  for  the  truth  of  this 
continuity  of  the  three  classes.  The  next  will  be  largely 
devoted  to  proving  it  to  you. 

§5. 

What  now  is  there  common  to  suspensions,  colloid  solu- 
tions and  molecular  solutions? 

Briefly  stated,  the  physical  and  chemical  properties  change 
in  periodic  fashion  in  all  three.  Let  us  imagine  a  suspension 
of  quartz  particles  in  water.  Were  we  to  measure,  by 
appropriate  means,  the  changes  in  the  coefficients  of  re- 
fraction in  such  a  suspension  and  plot  the  results,  we  should 
obtain  such  figures  as  are  shown  in  Figs.  3  and  4.  We 
should  encounter  a  periodic  increase  and  decrease  in  the 
coefficient  of  refraction  depending  upon  whether  we  were 
striking  a  quartz  particle  or  the  suspending  medium.  The 
same  periodic  change,  not  only  in  the  coefficients  of  refrac- 
tion but  in  all  other  physical  and  chemical  properties,  would 
be  encountered,  no  matter  in  which  direction  we  went 
through  the  quartz  suspension.  But  such  periodic  changes 
in  properties  would  also  be  encountered  in  any  solution  in 
which  the  substances  are  in  a  state  of  molecular  division. 


FUNDAMENTAL  PROPERTIES 


15 


In  molecular  solutions,  too,  must  appear  points  where  the 
physico-chemical  properties  of  the  solvent  predominate,  and 
again  others  in  which  the  properties  of  the  dissolved  mole- 
cules, combined  perhaps  with  the  solvent,  predominate. 


o       o       o       o 

o       o      o       o       o 

o       o       o       o 

o       o       o       o       o 

0 e e e — 


o  o  o  o  o 
o  o  o  o 

o  o  o  o  o 
o  o  o  o 


FIG.  3.  —  Diagram  illustrating  the  concept,  dispersed  system. 
Density,  Coefficient  of  Refraction,  etc. 


L... 


-^-Length  (in/x/t) 


y        t 

Particles 
FIG.  4.  —  Diagram  illustrating  the  concept,  dispersed  system. 

Thus  in  such  an  electrolyte  as  a  dilute  salt  solution  we  know 
that  the  positive  and  negative  electricities  must  follow  each 
other  in  succession.  Other  physico-chemical  properties, 
like  density,  must  change  similarly,  but  such  periodic 
changes  in  a  molecular  solution  must  occur  within  smaller 
spaces  (within  what  we  call  molecular  distances)  than  in  a 


16  COLLOID  CHEMISTRY 

quartz  suspension.  What  has  been  said  regarding  suspen- 
sions and  molecular  solutions  must  hold  for  colloid  solutions 
also.  In  all  three  the  physical  and  chemical  properties 
show  periodic  changes  in  space. 

§6. 

This  view  is  a  central  one  in  modern  colloid  chemistry. 
Colloid  chemistry  speaks  of  the  disperse  structure  of  these 
solutions,  applying  to  them  the  general  terms  dispersed 
systems  or  dispersoids.  A  dispersed  system  is  therefore 
nothing  more  than  one  in  which  the  properties  change 
periodically  in  space. 

To  address  myself  for  a  moment  to  the  physical  chemists 
among  you,  it  is  clear  that  this  definition  is  more  inclusive 
than  that  represented  by  the  terms  "polyphasic"  or  "  hetero- 
geneous." When  we  speak  of  a  polyphasic  system,  as 
represented,  for  instance,  by  a  quartz  suspension,  we  mean 
that  periodically  a  whole  series  of  properties  changes  at  once. 
Practically  all  the  physical  and  chemical  properties  change 
as  we  pass  from  the  one  phase  into  the  other.  But  our 
concept  of  dispersion  makes  no  assumptions  whatsoever 
regarding  either  the  kind  or  the  number  of  the  properties 
which  are  changed  in  space.  If  you  will  call  to  mind,  for 
example,  that  RONTGEN  rays  are  regarded  as  little  more 
than  oscillating  systems  of  electrically-charged  masses,  the 
electrons,  you  will  observe  that  it  is  possible  to  construct 
dispersed  systems  which  consist,  practically,  of  but  one  form 
of  energy.1 

The  term  dispersed  system  is  valid,  therefore,  not  only 
for  so-called  "  heterogeneous "  systems  but  also  for  "  homo- 
geneous7' ones,  as  represented  by  molecular  solutions.  The 
term  means  less  than  heterogeneity,  yet  it  contains  more 
than  the  term  homogeneity.  This  discussion  may  sound 

1  The  concepts  of  quantity  which  have  recently  been  applied  to  different 
kinds  of  energy  and  combinations  of  energy  are  adaptable,  in  large  measure, 
to  dispersed  systems,  provided  it  be  remembered  that,  as  ordinarily  used, 
the  term  "quantity"  embraces  only  units  possessing  a  maximal  degree  of 
dispersion. 


FUNDAMENTAL  PROPERTIES  17 

too  theoretical  to  please  you,  but  I  believe  that  you  will  soon 
recognize  for  yourselves  how  illuminating  it  is  when  applied 
to  the  problems  which  we  are  about  to  take  up,  and  how 
fruitful  are  its  practical  applications. 


Coarse  suspensions,  colloid  solutions  and  molecular  solu- 
tions are  all  to  be  regarded  as  dispersed  systems  and  to  be 
studied  together  under  this  common  heading.  But  how  do 
they  differ  from  each  other?  They  differ,  first  of  all,  in  the 
number  of  the  periodic  changes  encountered  in  the  unit 
volume.  As  is  readily  apparent,  the  number  of  periods, 
or  the  degree  of  dispersion,  increases  while  we  pass  from  the 
coarse  suspensions  through  the  colloids  to  the  molecular 
solutions,  as  shown  in  the  following  diagram:1 
DISPERSED  SYSTEMS. 


Coarse  Suspensions  Colloids  Molecular  Solutions 


Direction  of  increasing  degree  of  dispersion 

The  degree  of  "  subdivision "  of  physical  and  chemical 
properties  is  greatest  in  the  molecular  systems  and  least  in 
the  coarse  suspensions.  Molecular  systems  belong  to  the 
most  highly  dispersed,  coarse  suspensions  to  the  least  dis- 
persed systems.  Colloid  solutions  occupy  a  middle  posi- 
tion. There  is,  of  course,  not  the  slightest  reason  for 
assuming  that  any  sudden  change  occurs  in  degree  of  dis- 
persion as  we  pass  from  the  coarsely  dispersed  to  the  colloid 
systems,  or  from  these  to  the  molecular.  Not  only  is  there 
no  theoretical  reason  against  such  a  view,  but  there  is  no 
practical  one  either.  As  I  shall  show  you  in  detail  in  the 
next  lecture,  we  know  dispersed  systems  of  every  degree  of 
dispersion  in  nature. 

It  is  well,  perhaps,  to  give  you  some  concrete  illustrations 
of  this.  I  show  you  here  a  series  of  different  kinds  of  sulphur 

1  The  lecturer  will  obviously  not  write  this  diagram  anew  in  every  lecture 
but  simply  develop  it  from  a  single  diagram  mounted  once  and  for  all  time 
before  the  audience. 


18  COLLOID  CHEMISTRY 

(demonstration).  In  this  first  bottle  I  have  the  familiar 
large  yellowish-green  crystals;  their  structure  is  so  coarse 
that  we  can  hardly  speak  of  them  as  dispersed  systems.  In 
this  second  bottle  I  show  you  sticks  of  sulphur;  these  have 
a  crystalline  structure  but  the  crystals  are  already  so  highly 
dispersed  that  they  are  hardly  visible  to  the  naked  eye. 
This  third  bottle  contains  flowers  of  sulphur  which  represent 
under-cooled  droplets  of  sulphur  that  are  but  fractions  of  a 
millimeter  in  diameter;  they  show,  in  other  words,  a  micro- 
scopic degree  of  dispersion.  Here  I  show  you  colloid  sulphur 
in  a  watery  " dispersion  medium";  it  is  a  milky  liquid  from 
which  the  sulphur  separates  out  only  very  slowly;  in  a  drop 
of  it  placed  under  the  microscope  you  do  not  see  any  particles ; 
this  system  is  therefore  still  more  highly  dispersed  than  the 
preceding  one.  The  fifth  bottle  contains  another  largely 
colloid  sulphur,  namely,  sulphur  dissolved  in  benzol;  it  is  a 
scarcely  turbid,  yellowish  fluid  in  which  the  sulphur  is  still 
more  highly  dispersed  than  in  the  watery  solution.1  And 
here,  in  this  sixth  bottle,  I  show  you  molecularly  dispersed 
sulphur,  in  the  form  of  the  well-known  solution  of  sulphur 
in  carbon  disulphid. 

You  observe,  therefore,  how  one  and  the  same  substance 
may  appear  in  all  possible  degrees  of  dispersion.  Other 
substances  can,  of  course,  also  be  made  to  assume  different 
degrees  of  dispersion.  For  example,  there  are  the  large 
crystals  of  sodium  chlorid,  the  more  highly  dispersed  ones 
constituting  common  table  salt,  the  colloid  preparations  of 
sodium  chlorid  that  I  have  already  shown  you,  and  its 
ordinary  molecular  solution. 

But  these  facts  also  show  you  how  through  our  concept  of 
the  dispersed  system  our  main  problem  of  the  relation  of 
colloid  solutions  to  molecular  solutions  and  to  coarse  sus- 
pensions finds  a  simple  answer.  We  pass  from  the  one  into 
the  other  gradually  and  it  is  entirely  arbitrary  at  which 
point  we  decide  to  insist  on  lines  of  division  between  the 
three  classes.  On  theoretical  grounds  we  cannot  say  what 

1  See  J.  AMANN,  Kolloid-Zeitschr.,  8,  197  (1911). 


FUNDAMENTAL  PROPERTIES  19 

degree  of  dispersion  is  characteristic  of  any  one  of  the 
classes.  On  practical  grounds,  however,  we  can  settle  upon 
values  which  are  suitable  as  a  basis  for  division.  These 
coincide  with  degrees  of  dispersion  at  which  certain  methods 
used  in  the  investigation  of  dispersed  systems  either  fail  or 
can  first  be  used  to  advantage. 

58. 

A  practical  division  between  coarsely  dispersed  and  colloid 
systems  can  be  made,  for  instance,  microscopically.  It 
follows  from  the  theory  of  microscopic  vision  that  we  cannot 
see  individual  particles  of  a  diameter  of  less  than  half  a  wave 
length  of  light.  By  employing  micro-photographic  methods 
which  enable  us  to  work  with  the  short  waves  of  ultra-violet 
light,  we  obtain  as  the  limit  of  microscopic  vision  a  value  of 
about  one  ten- thousandth  of  a  millimeter  or  0.1  p.  This 
value  is  used  —  let  it  be  noted,  altogether  arbitrarily  —  as 
marking  the  transition  from  coarse  to  colloid  dispersions. 
Other  methods  of  investigation,  such  as  filtration,  yield 
similar  values.  The  pores  of  the  best  grades  of  hard  filter 
paper  (No.  602  of  Schleicher  and  Schull)  are  about  1  ^  in 
diameter;  those  of  clay  and  porcelain  filters  about  0.2  to 
0.4  /i.  These  values  therefore  approximate  those  obtained 
by  microscopic  means.  It  is  characteristic  of  typical  col- 
loids that  they  pass  through  these  filters  while  coarse  sus- 
pensions do  not. 

If  now  we  seek  a  line  for  the  division  of  colloids  from 
molecularly  dispersed  solutions,  we  may  begin  by  asking 
the  physical  chemists  about  the  size  of  molecules.  By 
methods  which  we  cannot  discuss  here,  they  have  decided 
that  typical  molecules  have  a  diameter  of  one  ten-millionth 
to  one  one-millionth  of  a  millimeter,  in  other  words  0.1  to 
1.0  MJU.  The  diameter  of  a  very  large  molecule  like  that  of 
starch  has  been  calculated  as  5  /z//.  But,  as  you  know, 
starch  dissolved  in  water  shows  marked  colloid  properties, 
so  this  value  comes  within  the  realm  of  colloid  dimensions. 
We  are  familiar  with  colloid-chemical  methods  to  be  dis- 


20  COLLOID  CHEMISTRY 

cussed  later,  like  optical  ones,  for  example,  which  also  begin 
to  fail  us  when  we  reach  dimensions  approximating  one  one- 
millionth  of  a  millimeter.  It  has,  therefore,  been  agreed, 
again  arbitrarily,  of  course,  to  draw  the  lines  between  colloid 
and  molecular  at  this  point. 

The  region  of  dispersity  within  which  the  colloids  lie  is 
therefore  bounded  by  particles  having,  on  the  one  hand,  a 
diameter  of  one  ten-thousandth  of  a  millimeter,  on  the  other 
one  of  one  one-millionth  of  a  millimeter,  as  indicated  in  the 
following  diagram: 

DISPERSED   SYSTEMS 


Coarse  Dispersions  Colloids  Molecular  Dispersoids. 

> 

Increase  in  degree  of  dispersion 
0.1  M        to        1.0  MM 


Periods  greater  than 
0.1  fj.',  do  not  pass 
through  paper  ni- 
ters; microscopi- 


cally analyzable. 


Pass  through  paper  filters; 
cannot  be  analyzed  micro- 
scopically; do  not  diffuse 


Periods  smaller  than 
1 .0  nn]  pass  through 
filter  paper,  cannot 
be  analyzed  -micro- 
scopically; diffuse 


or  cnalyze. 

ancf  diaiyze. 

Dispersed  systems  lying  within  these  middle  limits  are 
called  typical  colloids,  but  let  me  again  emphasize  that  we 
deal  with  purely  arbitrary  divisions  and  that  we  are  familiar 
with  transition  systems  of  every  degree  of  dispersity  not 
only  between  coarse  dispersions  and  colloids  but  between 
these  and  molecularly  dispersed  systems. 

§9. 

We  are  now  able  to  answer  the  question  raised  at  the 
beginning  of  this  lecture:  what  are  colloids?  According 
to  modern  colloid  chemistry  they  belong,  with  mechanical  sus- 
pensions and  molecular  solutions,  to  the  group  of  the  dispersed 
systems,  differing  from  the  suspensions  and  the  molecular  solu- 
tions only  in  the  special  value  of  their  degree  of  dispersion. 
This  is  the  theoretical  definition  of  the  colloids.  From  an 
experimental  point  of  view  —  and  under  this  heading  we 
shall  get  the  answer  to  our  questions  regarding  the  means 
by  which  we  may  recognize  colloids  —  the  colloids  differ 


FUNDAMENTAL  PROPERTIES  21 

from  the  coarse  dispersions  in  that  they  cannot  be  analyzed 
microscopically  and  in  that  they  pass  through  ordinary  filters. 
The  colloids  differ  from  molecularly  dispersed  systems  in  that 
they  do  not  diffuse  and  do  not  dialyze,  which  molecular  solu- 
tions do.  But  should  you  ever  find  occasion  to  express  or  to 
make  use  of  this  modern  definition,  do  not  forget  to  add  that 
there  exist  transition  systems  not  only  between  coarse  disper- 
sions and  colloids  but  between  colloids  and  molecularly  dis- 
persed solutions.  The  colloids  merely  represent  a  realm 
differentiated  for  practical  purposes  from  a  continuous  series 
of  systems. 

If  all  this  is  true,  some  important  corollaries  follow.  If 
colloids  are  " nothing  but"  systems  of  a  certain  sub-molec- 
ular degree  of  dispersion,  it  follows  that  every  substance 
may  appear  in  colloid  form  or  be  made  to  appear  so,  for, 
theoretically  at  least,  we  know  that  for  every  substance 
there  must  exist  a  second  substance  in  which  the  first  is  not 
spontaneously  soluble  in  molecular  form.  You  can  see  for 
yourselves  how  well  experience  bears  out  this  conclusion. 
T?ke  table  here  is  covered  with  a  fairly  large  number  of 
colloid  preparations  and  I  have  told  you  that  there  are  many 
hundred  others.  There  are  so  many  that  it  is  impossible  to 
list  them  all.  These  facts  are  the  best  sort  of  confirmation 
ofthe  teaching  that  every  substance  may  be  obtained  in 
colloid  form,  or  expressed  in  the  words  of  the  Russian 
investigator,  P.  P.  VON  WEIMARN:  The  colloid  state  is  a 
universally  possible  state  of  matter. 

§10. 

Our  diagram  of  the  dispersed  systems  also  enables  us  to 
predict  by  what  general  methods  a  given  substance  may  be 
brought  into  the  colloid  state.  There  are  two  such.  We 
may  begin  with  a  non-dispersed  or  coarsely  dispersed 
system  and  increase  its  degree  of  dispersion  until  colloid 
dimensions  are  reached,  or  we  may  start  with  a  molec- 
ular system  and  allow  the  molecules  to  combine,  aggre- 
gate or  condense  until  the  colloid  state  is  reached.  The 


22  COLLOID  CHEMISTRY 

former  of  these  is  known  as  the  dispersive,  the  latter  as  the 
condensation  method  of  producing  colloids. 

Many  different  methods,  or,  better  expressed,  many 
different  types  of  energy  may  be  used  either  to  disperse  or 
to  condense  the  molecules  of  any  substan.ce.  Colloids  may 
be  prepared  by  employing  not  only  mechanical  energy  but 
chemical  or  electrical  energy,  or  even  heat  and  light.  Of  the 
many  possible  methods  I  can  show  you  but  a  few.  I  shall 
show  you  a  chemical  condensation  method  and  an  electrical 
dispersion  method. 

A  particularly  interesting  colloid  is  that  of  gold  which  I 
have  already  showed  you  as  an  intensely  reddish-violet  or 
bluish  liquid.  This  colloid  gold  was  prepared  even  in  the 
days  of  the  alchemists  by  the  reduction  of  gold  salts  with  all 
kinds  of  organic  substances,  such  as  urine.  B.  J.  RICHTER, 
M.  FARADAY  and  many  other  investigators  have  since  then 
studied  it.  In  order  to  obtain  it  by  a  method  of  chemical 
condensation  I  begin  with  a  molecularly  or  ionically  dis- 
persed solution  of  gold  chlorid  to  which  sodium  bicarbonate 
has  been  added  until  neutral  to  litmus.  I  need  now  to 
reduce  the  gold  chlorid  to  metallic  gold,  but  this  must  be 
done  in  such  a  way  that  the  resulting  gold  remains  so  highly 
dispersed  as  not  to  exceed  colloid  dimensions.  As  you  know, 
gold  chlorid  may  be  reduced  by  many  different  kinds  of 
substances,  especially  organic  ones.  You  need  but  dip  your 
finger  into  the  solution  when  it  becomes  stained  a  bluish- 
violet  by  the  colloid  gold  produced  through  the  reducing 
action  of  the  organic  substances  contained  in  the  skin.  You 
are  frequently  told  in  colloid  chemistry  that  the  preparation 
of  fairly  stabile  colloid  gold  is  a  delicate  undertaking,  for 
which  not  only  the  purest  distilled  water  is  necessary  but 
accurate  quantitative  work  as  well.  If  these  things  are 
ignored,  red  gold  is  rarely  obtained,  but  violet  or  blue  gold 
appears  instead.  I  want  to  show  yoti  a  method  by  which 
we  can  always  obtain  red  colloid  gold  even  when  we  work 
but  roughly. 

This  Erlenmeyer  flask  contains  about  100  cc.  of  ordinary 


FUNDAMENTAL  PROPERTIES  23 

distilled  water.  I  add  a  few  drops  of  a  neutralized  one  per- 
cent solution  of  gold  chlorid,  and,  after  mixing,  a  few  drops 
of  a  very  dilute  solution  (about  0.1  percent)  of  tannin.  We 
need  not  work  any  more  accurately  than  this.  Only  not 
too  much  gold  chlorid  or  too  much  tannin  must  be  used. 
The  completed  mixture  should  be  practically  colorless 
(demonstration).  I  now  heat  it  over  a  Bunsen  burner  for 
one  or  two  minutes,  shaking  it  constantly.  You  observe 
that  even  before  the  mixture  begins  to  boil  it  assumes  a 
cherry  red  color.  I  may  now  add  more  gold  chlorid  or 
more  tannin  as  necessary  and  thus  obtain  an  almost 
reddish-black  solution.  The  experiment  will  succeed  even 
with  ordinary  tap  water.1 

You  may  be  interested  in  knowing  how  blue  or  violet  gold 
is  prepared.  A  method  which  furnishes  blue  gold,  as  cer- 
tainly as  the  previously  described  experiment  yields  red, 
consists  in  adding  a  few  drops  of  a  very  dilute  solution  of 
hydrazin  hydrochlorid  to  a  dilute,  neutral  solution  of  gold 
chlorid.  The  blue  color  appears  almost  immediately  if 
enough  of  the  reducing  material  is  added  (demonstration). 
If  I  add  but  little,  a  violet  color  is  obtained.  If  too  con- 
centrated solutions  are  employed,  the  gold  becomes  bluish- 
black  or  greenish-black.  It  is  then  no  longer  colloid  but 
precipitates  out  in  microscopically  visible  particles.2 

1  This  method  for  the  production  of  stabile  red  colloid  gold  is  interesting 
because  it  really  "works"  every  time,  provided  only  neutralized  gold  chlorid 
is  used  and  the  work  is  carried  out  in  sufficiently  great  dilutions.     If  the  red 
color  does  not  appear  immediately  after  heating,  more  tannin  and  more 
gold  chlorid  may  be  added  alternately  without  endangering  the  possibilities 
of  getting  the  desired  red  color.     It  must  be  cautioned  that  the  hot  solution 
may  not  at  once  be  diluted  with  cold  water.     This  is  likely  to  bring  about 
a  change  from  the  cherry  red  to  violet.     After  the  colloid  solution  has  been 
cooled,  dilution  will  not  markedly  affect  the  color.     This  assured  method 
for  producing  red  colloid  gold  (I  have  performed  the  experiment  countless 
times  with  all  kinds  of  materials  and  even  when  only  tap  water  was  at  hand) 
seems  not,  as  yet,  to  have  been  described  in  the  literature.     Its  success  seems 
to  depend  upon  the  fact  that  the  tannin  acts  not  only  as  a  reducing  sub- 
stance but,  to  a  certain  degree,  also  as  a  protective  colloid. 

2  This  method  also  always  "works."     A  tiny  crystal  dissolved  in  some 
20  cc.  of  water  yields  a  solution  which  for  most  purposes  acts  as  a  sufficiently 
strong  reducing  mixture. 


24  COLLOID  CHEMISTRY 

These  demonstrations  illustrate  chemical  condensation 
methods.  We  begin  with  molecularly  dispersed  systems, 
free  the  gold  molecules  and  then  allow  them  to  coalesce  into 
larger  aggregates.  We  choose  the  conditions  for  our  experi- 
ments so  that  the  aggregation  does  not  proceed  to  the  point 
of  yielding  coarsely  dispersed  precipitates  but  ceases  as  soon 
as  the  condensation  has  attained  colloid  dimensions.  What 
are  the  experimental  conditions  which  must  be  maintained 
hi  order  to  attain  this  end? 

You  have  seen  for  yourselves  that  I  have  worked  only  with 
dilute  solutions.  As  I  emphasized  in  the  experiment  on  the 
precipitation  of  colloid  mercuric  sulphid,  we  obtain  a  colloid 
precipitate  which  will  pass  through  the  filter  only  if  the 
precipitation  is  produced  in  very  dilute  solutions.  Let  me 
show  you  another  example  of  this  dependence  of  degree  of 
dispersion  of  the  precipitate  obtained  upon  the  concentra- 
tion of  the  reacting  solutions. 

I  have  poured  together  in  this  first  beaker  two  very  dilute 
solutions  of  iron  chlorid  and  potassium  ferrocyanid.  The 
resulting  precipitate  of  berlin  blue  is  so  highly  dispersed  - 
it  is  a  colloid  —  that  the  liquid  is  intensely  blue  yet  appears 
perfectly  clear  to  the  naked  eye  (demonstration).1  In  this 
second  beaker  I  have  poured  together  two  somewhat  more 
concentrated  solutions  of  the  same  materials.  You  observe 
that  a  bulky,  dark-blue  precipitate  has  formed,  above  which 
there  remains  the  slightly  colored  dispersion  medium. 
Evidently,  therefore,  the  degree  of  dispersion  is  less  in  this 
second  beaker,  while  the  size  of  the  individual  particles  of 
the  precipitate  has  been  increased,  simply  by  working 
with  more  concentrated  solutions  of  the  reacting  ma- 
terials. 

I  show  you  now  two  still  more  highly  concentrated, 
practically  saturated  solutions  of  the  two  reagents.  When 

1  For  demonstration  purposes  it  is  best  to  use  large  glass  cylinders  or 
parallel-walled  museum  jars.  These  should  be  lighted  from  behind  by 
means  of  an  arc  lamp,  the  light  from  which  is  made  to  pass  through  ground 
glass  or  paper. 


FUNDAMENTAL  PROPERTIES 


25 


I  pour  these  together l  and  stir  with  a  glass  rod  (demonstra- 
tion) you  note  a  remarkable  fact:  the  two  liquids  set  to 
form  a  cheesy  paste  so  stiff  that  I  can  turn  the  beaker  upside 
down  without  losing  its  contents  (demonstration).  Please 
recall  that  the  paste  resulted  from  the  mixture  of  two  mobile 
liquids  possessing  in  themselves  no  high  degree  of  viscosity. 
I  now  make  the  following  experiment :  when  I  take  some  of 
this  thick  precipitate  and 
stir  it  into  a  large  volume  of 
distilled  water  (demonstra- 
tion) I  again  obtain  a  clear 
blue  liquid  which  is  fairly 
stabile  and  which  is  also 
colloid,  as  I  can  prove  to 
you  by  filtering  it  (demon- 
stration). It  looks,  there- 
fore, as  if,  by  the  use  of 
very  highly  concentrated 
reaction  mixtures,  the  size 
of  the  precipitated  particles 
is  again  decreased.  This  is  really  the  case,  as  has  been  shown 
in  detail  by  P.  P.  VON  WEIMARN.  The  precipitate  is 
coarsest  when  medium  concentrations  are  employed.  The 
size  of  its  particles  decreases  both  with  decrease  or  increase 
in  concentration  of  the  reacting  substances.  A  curve 
illustrating  the  relation  of  size  of  particles  to  concentration 
of  the  reacting  solutions  would,  therefore,  show  a  maximum 
in  a  region  of  medium  concentration,  as  indicated  in  Fig.  5. 

Ill- 

Because  of  the  importance  of  this  VON  WEIMARN  law  in 
colloid  synthesis  by  chemical  condensation  methods,  I  show 
you  a  number  of  microphotographs  illustrating  these  facts 

1  Since  the  saturated  potassium  ferrocyanid  solution  contains  much  less 
salt  than  the  iron  chlorid  solution,  the  two  liquids  must  be  mixed  in  about 
the  proportions  of  two  to  one.  The  iron  chlorid  is  poured  into  the  cyanid  — 
not  the  other  way  about. 


Concentration  of-the  reaction  mixtui 
FIG.  5.  —  Influence  of  the  concentra- 
tion of  the  reaction  mixtures  upon  the 
size  of  particles  of  a  precipitate. 


26  COLLOID  CHEMISTRY 

(demonstration).1  The  pictures  are  photographs  of  barium 
sulphate  precipitates,  made  by  pouring  together  barium 
cyanid  and  manganese  sulphate.  They  show  the  changes 
in  the  character  of  the  precipitate  in  passing  from  mixtures 
of  dilute  solutions  to  those  which  are  more  concentrated. 
Fig.  6  presents  the  precipitate  formed  on  mixing  ^Vtf 
normal  solutions.  As  you  see,  the  picture  shows  nothing. 
This  is  just  what  it  is  intended  to  show.  At  this  concentra- 
tion we  obtain  a  colloid  precipitate  of  barium  sulphate, 
and,  since  colloid  particles  are  not  visible  microscopically, 
the  photograph  could,  of  course,  show  nothing.  Fig.  7, 
obtained  with  -foW  normal  solutions,  begins  to  show  par- 
ticles. The  enlargement  is  about  1  :  1500.  As  we  approach 
the  higher  concentrations  of  -%fa  normal  to  -£$  normal 
(Figs.  8  and  9)  we  observe  a  gradual  increase  in  the  size 
of  the  particles.  The  photographs  are  all  on  the  same  scale 
and  may,  therefore,  be  compared  directly  with  each  other. 
At  still  higher  concentrations,  ^  to  y1^  normal,  actual 
crystals  begin  to  appear,  as  evident  in  Figs.  10  and  11. 
In  these  concentrations  the  maximum  size  for  the  indi- 
vidual particles  has  been  attained.  From  this  point  on,  as 
we  pass  through  the  higher  concentrations  of  |,  J  and  ^ 
normal,  you  observe  that  the  size  of  the  particles  again 
decreases  steadily  (Figs.  12,  13  and  14).  In  still  higher 
concentrations,  such  as  f  to  \  normal  (Figs.  15,  16  and 
17)  we  obtain  the  paste-like  precipitates  which  I  showed 
you  in  the  case  of  berlin  blue.  The  pictures  of  such  precip- 
itates appear  as  solid  films  torn  in  various  ways.  It  is 
still  possible  to  make  out  that  these  films  consist  of  minute 
crystals.  In  the  most  highly  concentrated,  almost  satu- 
rated solutions  the  microphotograph  again  shows  nothing 
(Fig.  17). 
It  is  amusing  that  in  the  classic  German  formulas  for 

1  See  P.  P.  VON  WEIMARN,  Kolloid-Zeitschr.,  2  (1907,  1908);  see  also 
his  Zur  Lehre  von  den  Zustanden  der  Materie,  Dresden  and  Leipzig,  1914. 
Not  all  the  photographs  appearing  in  the  original  are  reproduced  herewith. 
For  lecture  purposes  it  is  best  to  project  diapositives. 


FUNDAMENTAL  PROPERTIES  27 

n 


FIG.  6. 


n 


FIG.  7. 


28 


COLLOID  CHEMISTRY 


FIG.  8. 


FIG.  9. 


FUNDAMENTAL  PROPERTIES  29 


B^*_--*fl«HBi 


FIG.  10. 


n 


FIG.  11 


30 


COLLOID   CHEMISTRY 


FIG.  12. 


FIG.  13. 


FUNDAMENTAL  PROPERTIES  31 

SBI3B3^BBS^S^B3£i£S5i^^^B^^^^^^^^^^^^^^^^^^B^^5'^5^^^r^  £  '3*&&tl--i 

f 


FIG.  14. 


•H^^^nuM^^n^M^^tf^H  '  ! 


FIG.  15. 


32  COLLOID  CHEMISTRY 

n 


FIG.  16. 


n 


FIG.  17. 


FUNDAMENTAL  PROPERTIES 


33 


making  colloid  solutions  through  chemical  condensation, 
very  dilute  solutions  are  always  insisted  upon  while  the 
formulas  of  the  American  colloid  chemist  CAREY  LEA  are 
equally  insistent  upon  the  use  of  concentrated  ones.  In 
keeping  with  the  American  way  of  doing  things  CAREY 
LEA'S  formulas  begin  by  calling  for  several  grams  of  gold 
chlorid. 

§12. 

I  show  you  next  a  dispersion  method  of  producing  colloid 
solutions  in  which  use  is  made  of  electrical  energy.     It  is 


FIG.  18.  —Apparatus  for  dispersing  metals  electrically. 

G.  BREDIG'S  method  of  dispersing  metals.  I  have  here  two 
silver  wires  fastened  into  a  stand  in  such  a  way  that  the 
ends  may  be  approximated  by  turning  a  micrometer  screw 
(Fig.  18).1  A  five-  to  eight-ampere  current  obtained  by 
sending  the  ordinary  110-volt  current  through  a  rheostat 
is  now  sent  through  these  wires.  Their  tips  dip  into  dis- 
tilled water  which  has  been  slightly  alkalinized  with  a  trace 
of  sodium  bicarbonate.  I  turn  on  the  current,  and  by 

1  This  exceedingly  useful  arrangement  was  placed  at  my  disposal  by 
Professor  J.  STIEGLITZ  in  the  University  of  Chicago. 


34  COLLOID  CHEMISTRY 

regulating  the  micrometer  screw,  allow  a  tiny  arc  to  form 
between  the  wire  tips  under  the  water  (demonstration). 
You  observe  how  dense  dark  brown  or  greenish  clouds 
emanate  from  the  tips  of  the  wires  and  remain  suspended  in 
the  water. 

This  greenish-brown  liquid  is  one  of  colloid  silver  some- 
what contaminated  with  colloid  silver  hydroxid.  The  solu- 
tion is  perfectly  clear  to  the  naked  eye  and  passes  easily 
through  filter  paper.  Other  metals  may  be  colloidally  dis- 
persed in  the  same  way;  in  fact  by  making  use  of  special 
methods  such  as  oscillating  electrical  discharges,  low  tem- 
peratures and  organic  dispersion  media,  THE  SVEDBERG 
has  prepared  even  the  alkali  metals  in  the  form  of  beauti- 
fully colored  colloid  solutions. 

We  can  also  prepare  colloid  solutions  by  exposing  metal 
plates  to  ultraviolet  light,  by  heating  alloys  and  cooling 
them  suddenly  in  water  and  by  grinding  powders  for  long 
periods  of  time.1 

§13. 

The  main  conclusions  then  to  which  this  lecture  has  led 
may  be  summarized  as  follows: 

Colloids  are  dispersed  systemSj  in  which  the  diameter  of  the 
dispersed  particles  in  typical  cases  lies  between  one  ten-thous- 
andth and  one  one-millionth  of  a  millimeter.  They  are  dis- 
tinguished experimentally  from  molecularly  dispersed  systems 
by  the  fact  that  they  do  not  dialyze;  and  from  coarse  dispersions 
by  the  fact  that  they  cannot  be  analyzed  microscopically.  Col- 
loids pass  through  filters  readily,  while  coarse  dispersions  do 
not.  Transition  systems  exist  between  colloids  and  molecular 

1  For  a  discussion  of  colloid  synthesis  through  exposure  to  radiant  energy 
see  WOLFGANG  OSTWALD,  Grundriss  d.  Kolloidchemie,  1.  Aufl.,  302,  Dresden, 
1909;  THE  SVEDBERG,  Kolloid-Zeitschr.,  6,  129,  238  (1910);  for  the  prepa- 
ration of  vanadic  acid  by  sudden  cooling  see  E.  MULLER,  Kolloid-Zeitschr., 
8,  302  (1911);  for  the  preparation  of  colloids  by  trituration  see  WOLFGANG 
OSTWALD,  Grundriss  der  Kolloidchemie,  1.  Aufl.,  292,  Dresden,  1909;  see 
also  C.  BENEDICKS,  Kolloidchem.  Beih.,  4,  260  (1913),  who  describes  the 
production  of  colloid  gold  by  trituration  as  practiced  in  the  seventeenth 
century,  as  well  as  G.  WEGELIN,  Koll.  Zeitschr.,  14,  65  (1914). 


FUNDAMENTAL  PROPERTIES  35 

solutions  and  between  colloids  and  coarse  dispersions.  The 
colloid  state  represents  a  universally  possible  state  of  matter. 
There  is  no  reason  why  every  substance  may  not  be  produced  in 
colloid  form.  It  may  be  accomplished  either  through  the  dis- 
persion of  non-dispersed  or  coarsely  dispersed  substances,  or 
through  the  condensation  of  molecularly  dispersed  systems. 
To  these  ends  not  only  chemical  but  mechanical,  electrical  and 
other  kinds  of  energy  may  be  used. 


II. 

CLASSIFICATION   OF  THE   COLLOIDS. 

THE  PHYSICO-CHEMICAL  PROPERTIES  OF  THE 

COLLOIDS  AND  THEIR   DEPENDENCE  UPON 

THE  DEGREE  OF  DISPERSION. 


SECOND  LECTURE. 

CLASSIFICATION  OF  THE  COLLOIDS.    THE 
PHYSICO-CHEMICAL  PROPERTIES  OF  THE 
COLLOIDS  AND  THEIR  DEPENDENCE 
UPON  THE  DEGREE  OF  DISPERSION. 

THE  previous  lecture  dealt  with  the  fundamental  facts 
and  problems  of  colloid  chemistry.  I  tried  to  show  you 
how  the  concept  of  the  colloid  has  assumed  a  new  meaning 
by  having  been  grouped  with  the  dispersed  systems.  The 
colloids  are  dispersed  systems  distinguished  by  the  special 
value  of  their  degree  of  dispersion.  This  view  emphasizes, 
in  other  words,  that  there  are  no  sharp  differences  between 
coarse  suspensions,  colloids  and  molecular  dispersoids.  We 
pass  gradually  from  one  into  the  other,  and  their  properties 
change  as  smoothly.  It  is  the  purpose  of  today's  lecture  to 
prove  the  truth  of  this  principle  of  continuity. 

I! 

Let  me  first  direct  your  attention  to  a  further  corollary  to 
the  conclusion  that  colloids  represent  dispersed  systems  in 
which  the  degree  of  dispersion  has  a  special  value.  You 
have  already  seen  how  this  modern  definition  compels  the 
conclusion  that  every  substance  may  appear  in  a  colloid 
state,  and  how  it  systematizes  also  the  general  methods  by 
which  colloids  may  be  prepared.  The  basis  for  a  third 
conclusion  may  be  introduced  as  follows. 

I  have  here  a  coarse  suspension  of  infusorial  earth  in  water 
(demonstration).  As  you  know,. infusorial  earth  consists  of 
the  silicic  acid  coverings  of  minute  organisms.  There  is  no 
doubt,  of  course,  that  this  mixture  is  one  of  solid  particles  in 
water.  The  same  is  true  of  this  black  precipitate  of  gold 
made  by  adding  more  gold  chlorid  and  more  reducing  agent 

39 


40  COLLOID  CHEMISTRY 

to  the  blue  colloid  gold  I  showed  you  yesterday  (demonstra- 
tion). The  particles  of  gold  in  blue  and  red  colloid  gold 
must  also  be  solid,  for  we  cannot  think  of  gold  at  ordinary 
temperatures  as  existing  in  any  other  form.  Coarse  dis- 
persions of  solid  particles  in  a  liquid  are  known  as  suspen- 
sions; colloid  dispersions  of  the  one  in  the  other,  as  suspension 
colloids  or  suspensoids. 

§2. 

I  show  you  in  this  flask  two  liquids  which  hardly  mix  with 
each  other  in  molecular  form,  namely,  water  and  benzol;  I 
have  added  a  little  iodin  to  the  latter  to  give  it  a  violet  color 
(demonstration).  When  I  shake  the  flask  you  observe  that 
I  obtain  mixtures  of  the  one  in  the  other,  in  other  words, 
two  emulsions,  one  of  benzol  in  water  and  another  of  water 
in  benzol.  We  have  here  divided  two  liquids  into  each 
other.  As  you  know,  this  subdivision  of  two  liquids  into 
each  other  can  be  carried  very  much  further,  as  seen  in  the 
milk  of  animals  and  plants,  in  cod  liver  oil  emulsions,  etc. 
The  mixture  may  be  so  highly  dispersed  that  high  power 
microscopes  are  necessary  to  distinguish  the  separate  drop- 
lets. This  is  true,  for  example,  of  human  milk  and  of  the 
milk  of  some  rubber  plants.  Do  we  know  emulsions  of 
a  colloid  degree  of  dispersion?  There  are,  of  course,  no 
reasons  against  the  existence  of  such  colloid  emulsions  or 
emulsoids,  for  we  know  that  liquids  dissolve  in  liquids  and 
the  principle  of  continuity  underlying  our  classification  of 
the  dispersed  systems  clearly  indicates  that  colloid  emul- 
sions must  exist  between  the  extremes  of  coarse  dispersions 
and  of  molecular  dispersions  of  one  liquid  in  another.  I 
show  you  here  two  types  of  such  colloid  emulsions  or,  to  use 
the  technical  term,  of  such  emulsion  colloids  or  emulsoids. 
As  an  example  of  the  first,  I  show  you  colloid  sulphur 
(demonstration).  As  even  the  older  investigators  knew, 
droplets  of  liquid  under-cooled  sulphur  are  formed  which 
slowly  solidify  or  crystallize  whenever  sulphur  is  precipi- 
tated in  watery  solution.  We  have  many  reasons  for 


CLASSIFICATION  OF  THE  COLLOIDS  41 

believing  that  the  microscopic  and  even  colloid  particles 
found  in  such  mixtures  retain  this  liquid  form.1 

In  illustration  of  this  second  type  of  liquid-liquid  colloids 
I  could  show  you  many  examples,  in  fact,  these  are  probably 
the  best  known  and  most  widely  distributed  of  all  the  col- 
loids. Glue,  gelatin,  agar,  protein,  starch,  rubber  and 
collodion  belong  to  this  group.  We  shall  discuss  it  in  detail 
in  the  next  lecture,  when  we  shall  also  take  up  the  differences 
existent,  for  example,  between  an  emulsoid  of  sulphur  and 
one  of  gelatin. 

§3. 

I  have  in  this  third  flask  an  example  of  another  coarsely 
dispersed  system,  a  coarse  dispersion  of  a  gas  in  a  liquid 
(demonstration).  You  see  before  you  a  saponin  foam. 
There  is  no  reason  why  a  gas  cannot  assume  a  dispersed  form. 
Are  there  dispersions  of  gases  in  liquid  in  which  the  degree 
of  dispersion  attains  colloid  dimensions?  No  doubt  there 
are,  for  we  are  familiar  both  with  coarse  dispersions  and 
with  molecular  dispersions  of  gases  in  liquids,  but  in  illustra- 
tion of  them  we  can  cite  but  few  examples.2  They  are  seen 
hi  the  opalescent  critical  mixtures  observed  when  liquids 
are  evaporated,  or  gases  are  being  liquefied  in  regions  of 
critical  temperature  and  pressure. 

§4. 

Certain  objections  may  be  raised  to  this  classification  of 
the  colloids  based  on  the  state  of  aggregation  of  their  con- 
stituents. The  term  itself,  state  of  aggregation,  refers  to 
material  in  mass.  It  evidently  loses  its  meaning  as  we 
approximate  the  molecular  dispersoids  in  our  progress 
through  the  series  of  dispersed  systems.  We  cannot  speak 
of  the  state  of  aggregation  of  a  molecule.  But  how  about 
colloid  particles?  May  these  still  exhibit  different  states 
of  aggregation?  Our  diagram  of  the  dispersed  systems  and 

1  See  the  monograph  of  S.  ODEN,  Der  kolloide  Schwefel,  Upsala,  1913. 

2  For  some  remarks  regarding  highly  dispersed  foams,  see  WOLFGANG 
OSTWALD,  Kolloid-Zeitschr.,  1,  333  (1907). 


42  COLLOID   CHEMISTRY 

our  definition  of  the  colloids  show  that  we  may  still  speak  of 
the  state  of  aggregation  of  colloid  particles.  The  individual 
particles  of  a  typical  colloid  must  certainly  consist  of  a 
whole  series  of  molecules.  As  we  reach  the  more  highly 
dispersed  among  the  colloids  the  properties  peculiar  to  any 
given  state  of  aggregation  must  gradually  disappear.  The 
properties  of  solid,  liquid  and  gaseous  particles  must,  in 
other  words,  become  more  and  more  alike.  This  is  a 
necessary  conclusion  from  the  principle  of  continuity  ex- 
pressed in  our  diagram  of  the  dispersed  systems,  for  a 
molecular  solution,  for  instance,  of  acetic  acid  in  water,  does 
not  betray  whether  it  was  produced  through  the  solution  of 
solid,  liquid  or  gaseous  acetic  acid  in  it.  It  is  evident  that 
we  may  expect  to  encounter  interesting  transition  phe- 
nomena in  this  realm. 

These  remarks  will  serve  to  indicate  how  broad  is  the 
field  of  the  colloids  when  the  different  states  of  aggregation 
and  their  possible  combination  into  dispersed  systems  are 
considered.  Thus  far  we  have  dealt  only  with  the  sub- 
division of  a  material  in  a  liquid  dispersion  medium.  But 
the  dispersion  medium  might,  of  course,  be  solid  or  gaseous. 
When  all  this  is  borne  in  mind,  the  following  eight  combi- 
nations become  possible  in  which  the  dispersed  material 
or  dispersed  phase  is  named  first,  the  dispersion  medium 
second. 

Solid  +  solid          Solids  +  liquids  (suspensoids)          Solid  +  gas  (smoke) 
Liquid  +  solid         Liquid  +  liquid  (emulsoids)  Liquid  +  gas  (fog) 

Gas  +  solid  Gas  +  liquid  (foams) 

It  is  important  to  emphasize  that  examples  of  coarse 
dispersions,  of  colloid  dispersions  and  of  molecular  dis- 
persions are  known  to  us  under  all  these  different  headings, 
although  the  coarse  dispersions  and  the  molecular  disper- 
sions are,  for  the  most  part,  the  more  familiar  examples. 

Many  of  the  minerals,  the  very  important  alloys,  the 
solid  solutions  of  VAN'T  HOFF,  etc.,  belong  to  the  system 
solid  +  solid.  As  colloid  examples  of  the  class,  I  show  you 
some  blue  rock  salt  (colloid  sodium  in  sodium  chlorid)  and 


CLASSIFICATION  OF  THE  COLLOIDS  43 

ruby  glass  (colloid  gold  in  glass).  Examples  of  the  sub- 
division of  a  liquid  into  a  solid  dispersion  medium  may  also 
be  found  in  mineralogy,  as  in  the  occlusion,  inclusion  and 
crystallization  waters  found  in  all  degrees  of  dispersion  in 
rocks.  Systems  illustrative  of  the  type  solid  -f-  gas  are 
meerschaum,  pumice,  lava  and  solutions  of  gases  in  metals. 
Gaseous  colloids  with  a  solid  dispersed  phase  are  seen  in 
tobacco  smoke,  cosmic  dust,  the  vapors  of  ammonium 
chlorid,  etc.  Systems  of  the  composition  gas  +  liquid  are 
seen  in  fogs  of  all  kinds,  as  in  the  fogs  about  the  earth,  and 
in  the  clouds  of  the  sky. 

This  list  will,  perhaps,  impress  you  not  only  with  the 
vastness  of  the  general  subject  of  the  dispersed  systems  but 
with  the  extent  to  which  these  dispersions  are  of  colloid 
dimensions.  The  modern  concept  of  the  colloid  has  served 
to  bring  together  under  one  head  many  scattered  elements. 
Where  in  the  realms  of  physical  chemistry  could  we  formerly 
place  the  foams  and  the  emulsions?  These  homeless  and 
yet  technically  important  structures  are  now  not  only 
properly  cared  for  but  are  seen"  to  be  of  the  greatest  signifi- 
cance in  the  science  of  colloid  chemistry. 


We  come  now  to  the  main  theme  of  today's  lecture.  We 
are  to  show  that  transition  phenomena  mark  our  passage 
from  the  coarse  dispersions  into  the  colloids,  and  from  these 
into  the  molecular  dispersoids.  I  shall  combine  with  this 
a  more  detailed  discussion  of  the  physical  and  chemical 
properties  of  colloid  systems.  To  this  end  I  shall  demon- 
strate the  mechanical,  optical,  electrical  and  physico- 
chemical  properties  of  some  colloid  solutions  while  showing 
you  at  the  same  time  how  these  change  as  we  pass  through 
the  different  degrees  of  dispersion.  Today's  subject  might 
be  called  the  physico-chemical  properties  of  dispersed  systems 
and  their  variation  with  the  degree  of  dispersion. 

Let  us  first  consider  some  of  the  mechanical  properties  of 
dispersed  systems.  If  you  examine  microscopically  a  fine 


44 


COLLOID  CHEMISTRY 


suspension  of  carmine  particles  in  distilled  water,  you 
observe  that  the  particles  are  in  a  state  of  spontaneous 
movement;  they  dance  and  rotate  in  an  apparently  irregu- 
lar manner,  the  individual  particles  following  such  paths  as 
have  been  plotted  in  Fig.  19.1  These  movements  are  not 
induced  through  an  expenditure  of  light  or  heat  energy  nor 
are  they  dependent  upon  electrical  or  chemical  changes. 
Moreover,  all  known  dispersed  systems  show  this  so-called 
BROWNIAN  movement  whenever  two  conditions  are  satisfied. 
First,  the  dispersed  particles  must  be  sufficiently  small.  As 
a  rule  BROWNIAN  movement  does  not  manifest  itself  clearly 


FIG.  19.  —  "  Paths  "  of  two  particles  in  BROWNIAN  motion. 

until  the  particles  have  a  diameter  of  0.5  /z  or  less.  Second, 
the  dispersion  medium  must  be  sufficiently  mobile  to  permit 
the  movement.  The  movements  could  not  appear  in  solid 
glass,  for  instance.  But  if  these  two  conditions  are  satisfied, 
all  dispersed  systems  show  BROWNIAN  movement.  It  is 
observed,  for  example,  in  milk,  in  gas  bubbles  and  very 
beautifully  in  smoke.  It  seems,  therefore,  to  be  a  general 
property  of  dispersed  systems  and  under  constant  conditions 
is  apparently  everlasting.  BROWNIAN  movement  is  ob- 
served in  the  liquid  inclusions  found  in  minerals  which  are 
certainly  thousands  of  years  old. 

How  does  this  BROWNIAN  movement  change  with  changes 
in  the  degree  of  dispersion?  Do  we  observe  a  BROWNIAN  or 
similar  movement  in  colloids  and  in  molecular  solutions? 

1  I  was  in  the  habit  of  concluding  the  lecture  by  demonstrating  ultra- 
microscopic  apparatus  and  with  this,  BROWNIAN  movement. 


CLASSIFICATION  OF  THE  COLLOIDS  45 

I  have  already  told  you  that  BROWNIAN  movement  of  micro- 
scopically visible  particles  is  observed  only  when  these  are 
highly  dispersed.  The  intensity  of  the  movement  increases 
markedly  as  the  microscopic  particles  decrease  in  size.  Our 
concept  of  continuity  would  lead  us  to  conclude  that  such 
movement  must  appear  in  colloids  and  molecular  solutions 
also,  and  since  the  degree  of  dispersion  is  greater,  the  move- 
ment would  occur  much  more  rapidly  here  than  in  micro- 
scopic dispersions.  There  no  doubt  now  comes  to  your 
mind  the  old  and  much-discussed  belief  that  in  all  molecular 
systems,  as  in  gases  and  liquids,  the  molecules  are  in  a  state 
of  marked,  even  tumultuous,  activity.  The  famous  kinetic 
theory  of  gases  and  liquids  is  built  upon  this  fundamental 
assumption.  It  can  be  shown  by  optical  methods,  which 
we  shall  discuss  later,  that  spontaneous  movement  not  only 
occurs  in  colloids,  but  is  more  intense  in  them  than  in  micro- 
scopic dispersions.  It  has  been  possible  to  show  that  this 
greater  velocity  of  BKOWNIAN  movement  approximates  the 
values  calculated  for  the  speed  of  molecules.  Conversely, 
the  laws  which  have  been  formulated  for  the  kinetic  move- 
ment of  molecules  hold  also  for  the  BROWNIAN  movement 
of  colloids  and  coarse  dispersions  if  their  degree  of  dispersion 
is  duly  considered.  We  shall  return  to  this  subject  when 
we  come  to  discuss  the  scientific  applications  of  colloid 
chemistry.  No  physical  chemist  today  questions  the  cor- 
rectness of  the  statement  that  this  "  spontaneous  internal 
movement"  is  common  to  all  dispersoids  and  that  its 
intensity  increases  steadily  as  we  pass  from  the  coarse  dis- 
persions, on  the  one  hand,  through  the  colloids,  to  the 
molecular  solutions  on  the  other. 

§6. 

Let  us  now  consider  another  mechanical  property  of  dis- 
persed systems.  A  particularly  important  qualitative  char- 
acteristic of  colloid  solutions  is  their  failure  to  diffuse  and 
to  dialyze.  Typical  colloids  do  not  diffuse  any  more  than 
do  coarse  dispersions.  Are  there  transition  systems  which 


46  COLLOID  CHEMISTRY 

occupy  a  position  between  the  colloid  and  the  molecular 
systems,  or  —  and  this  would  be  a  particularly  pretty 
proof  —  can  we  make  one  and  the  same  substance  appear 
at  one  time  in  diffusible  and  at  another  in  non-diffusible 
form?  An  experimental  answer  can  be  given  to  both  these 
questions.  We  know  many  solutions  which  assume  this 
intermediate  position  so  far  as  diffusion  is  concerned.  Many 
proteins,  ferments,  toxins,  antitoxins  and  dyes,  such  as 
congo  red,  night  blue,  etc.,  show  a  hardly  measurable  but 
nevertheless  definite  diffusibility.  Slight  changes  in  the 
dispersion  media  suffice  at  times  to  impart  to  these  transi- 
tion systems  a  well-marked  diffusibility.  Thus  certain 
albumins  do  not  diffuse  into  distilled  water  but  diffuse 
readily  into  dilute  salt  solutions.  The  neutral  salts  dehy- 
drate the  heavily  hydrated  colloid  albumin  particles,  thereby 
increasing  their  dispersion  and  so  their  diffusibility.1  All 
degrees  of  diffusibility  are  encountered  in  passing  from  the 
molecularly  dispersed  to  the  colloid  solutions. 

But  even  one  and  the  same  substance  in  a  given  dispersion 
medium  without  any  additions  from  the  outside,  may  either 
diffuse  or  not,  depending  upon  its  degree  of  dispersion. 

1  Papers  dealing  with  this  subject  hardly  discuss  the  fact  that  the  addition 
of  a  neutral  salt  or  of  alcohol  to  a  hydrated  colloid  may  bring  about  two 
totally  different,  antagonistic  effects.  First,  addition,  of  these  foreign  ma- 
terials increases  the  degree  of  dispersion  by  dehydrating  the  particles;  through 
secondary  agglomeration  of  the  particles  there  then  occurs  a  decrease  in 
degree  of  dispersion  which  inay  end  in  coagulation.  According  to  unpub- 
lished experiments  of  my  own,  this  double  effect  is  separable  by  proper 
methods,  and  explains,  for  example,  the  formerly  unintelligible  fact  that 
protein  solutions  diffuse  more  readily  into  dilute  salt  solutions  than  into 
distilled  water.  See  WOLFGANG  OSTWALD,  Handbook  of  Colloid  Chemistry, 
translated  by  FISCHER,  OESPER  and  BERMAN,  Philadelphia,  1915.  It  also  ex- 
plains why  colloid  dyes  like  congo  red  upon  the  addition  of  salts  first  turn 
towards  yellow  and  only  later,  shortly  before  precipitation,  towards  violet. 
The  fact  that  large  amounts  of  neutral  salt  must  be  present  to  accomplish 
the  crystallization  of  proteins  is  also  to  be  explained  by  the  influence  of  the 
neutral  salts  in  bringing  about  a  decrease  in  the  degree  of  dispersion  of  the 
hydrated  colloid.  It  is  presumable  that  the  crystalline  or  vectorial  forces 
of  the  particles  will  come  into  action  best  when  the  amount  of  indifferent  so- 
lution medium  bound  to  the  particles  and  tending  to  inhibit  their  coalescence, 
is  least. 


CLASSIFICATION  OF  THE  COLLOIDS  47 

This  was  proved  years  ago  by  W.  RAMSAY'S  pupils,  H. 
PICTON  and  S.  E.  LINDER,  for  the  precipitates  of  arsenic 
trisulphid.  In  keeping  with  the  law  of  VON  WEIMARN  these 
authors  obtained,  from  very  dilute  solutions,  precipitates  of 
arsenic  trisulphid  which  were  not  only  invisible  under  the 
microscope  and  passed  a  filter,  but  showed  undoubted  dif- 
fusibility.  Similar  observations,  according  to  my  experi- 
ence, may  be  made  on  the  highly  dispersed  colloids  of  berlin 
blue  and  according  to  THE  SVEDBERG  on  colloids  of  gold. 
In  fact,  in  gold  the  connection  between  degree  of  dispersion 
and  diffusibility  seems  so  simple  that  the  diffusion  coefficient 
appears  as  inversely  proportional  to  the  diameter  of  the 
particles.1  This  constitutes,  moreover,  a  quantitative  con- 
clusion derived  from  application  of  the  kinetic  theory  to 
these  more  coarsely  dispersed  systems.  All  these  facts 
leave  no  room  to  doubt  that  diffusibility  and  therefore 
dialyzability  increase  progressively  as  the  degree  of  dispersion 
increases,  just  as  in  BROWNIAN  movement. 

§7. 

If  we  would  discover  examples  of  transition  phenomena 
in  the  mechanical  properties  of  coarse  dispersions  and  of 
colloids  we  may  study  their  behavior  during  filtration.  We 
may  recognize  different  degrees  of  dispersion  as  they  will  or 
will  not  pass  through  filters  of  a  known  porosity.  To  give 
you  some  idea  of  the  size  of  the  pores  in  different  filters,  I 
show  you  the  following  table  of  H.  BECHHOLD. 

SIZE  OF  PORES  IN  FILTERS 

Ordinary  thick  filter  paper About  3.3^. 

Filter  paper  No.  556  (Schleicher  and  Schull) About  1 .7  /*. 

Filter  paper  No.  602  extra  hard  (Schleicher  and  Schull)  About  0 . 89  to  1 . 5  /z. 

Chamberland  filter About  0.23  to  0.41  /*. 

Reichel  filter About 0. 16toO.  175  M. 

In  keeping  with  our  definition,  colloids  would,  therefore, 
be  held  back  only  by  the  fine  porcelain  filters. 

C.  J.  MARTIN,  G.  MALFITANO  and,  in  recent  years,  H. 

1  See  THE  SVEDBERG,  Zeitschr.  f.  physikal.  Chem.,  67,  105  (1909). 


48 


COLLOID  CHEMISTRY 


BECHHOLD,  have  taught  us  how  to  make  filters  which  enable 
us  to  separate  colloids  from  their  dispersion  media.  We 
shall  soon  see,  as  a  matter  of  fact,  that  filters  may  be  pre- 
pared which  will,  in  part  at  least,  bring  about  a  mechanical 
separation  of  dispersed  phase  from  solvent  even  in  the  case 
of  the  molecular  dispersoids.  Certain  organic  and  inorganic 
gels  come  under  this  heading.  Thus,  when  colloids  are 
filtered  through  collodion  sacs  (under  pressure  when  neces- 
sary) it  is  often  possible  to  separate  the  colloid  from  its 
dispersion  medium.  I  show  you  here  an  example  of  this. 
I  have  poured  into  the  collodion  sac  shown  in  Fig.  20  a 
dark  brown  solution  of  colloid  silver. 
Nothing  passes  through  the  filter  but  a 
practically  colorless  liquid  (demonstra- 
tion).1 This  filtration  of  colloids  we 
call  ultrafiltration.  We  can  so  vary 
the  permeability  of  the  ultrafilters 
through  the  addition  of  various  sub- 
stances or  by  changing  their  concen- 
tration that  from  a  given  colloid  we 
may  obtain  fractions  differing  from 
each  other  in  the  degree  of  their  dis- 
persion. Concentrated  gels  hold  back 
\^  ^/  even  the  most  highly  dispersed  colloids. 

FIG.  20.— Simple  arrange-  if  we  use  inorganic  gels  of  the  type  of 

ment  for  ultrafiltration.      ^^    U1 ^    by   mixing   together 


highly  concentrated  solutions,  we  obtain  the  so-called  semiper- 
meable  membranes  used  in  osmotic  experiments.  These  jelly- 
like  precipitates  may  be  so  impermeable,  as  in  the  case  of  cop- 
per f  errocyanid,  that  they  will  not  give  passage  even  to  many 
dissolved  molecules.  We  can  make  use  of  this  property  not 
only  to  bring  about  changes  in  the  concentration  of  molecu- 

1  The  simplest  arrangement  for  such  an  ultrafiltration  experiment  is 
probably  that  of  SCHOEP  [Koll.-Zeitschr.,  8,  80  (1911)].  In  order  to  hasten 
the  ultrafiltration,  the  funnel,  to  which  is  attached  the  collodion  sac,  may 
be  pushed  through  a  rubber  stopper  and  the  whole  set  into  a  filtration  flask, 
the  side  tubulation  of  which  is  connected  with  a  water  pump,  as  shown  in 
Fig.  20  in  the  text. 


CLASSIFICATION  OF  THE  COLLOIDS  49 

lar  solutions  in  which  the  osmotic  pressure  assumes  the  role 
of  filtration  pressure,  but  to  bring  about  a  separation  of  the 
solid  salt  in  highly  concentrated  solutions.  Thus,  accord- 
ing to  the  physiologist  C.  LUDWIG,  a  fairly  concentrated 
solution  of  sodium  sulphate  begins  to  crystallize  when  a 
piece  of  dried  pig's  bladder  is  introduced  into  it,  for  only 
water  and  not  salt  diffuses  into  this  concentrated  gel.  All 
this  again  serves  to  show  that  we  pass  by  easy  steps  from 
ordinary  filtration  through  ultrafiltration  to  osmotic  or 
"superultrafiltration"  (P.  P.  VON  WEIMARN). 

§8. 

When  we  consider  the  optical  properties  of  colloid  systems 
we  again  encounter  a  large  number  of  beautiful  and  inter- 
esting transition  phenomena.  Perhaps  the  most  general 
optical  phenomenon  encountered  in  dispersoids  is  that  of 
optical  heterogeneity,  or  turbidity.  A  dispersed  material 
and  the  dispersion  medium  will  ordinarily  hardly  be  expected 
to  show  the  same  coefficient  of  refraction.  A  ray  of  light 
passing  through  the  system  will,  therefore,  not  be  able  to 
do  so  undisturbed.  This  is  the  scientific  explanation  of 
turbidity,  so  well  shown  by  coarse  and  microscopic  disper- 
sions of  all  kinds.  I  need  but  remind  you  of  the  white  color 
of  quartz  suspensions,  of  milk  and  of  foam.  But  a  large 
number  of  colloids  also  appear  turbid  if  properly  studied, 
as  these  colloid  metallic  sulphids,  this  bluish-black  colloid 
gold,  etc. 

We  can  best  perceive  slight  turbidities  by  viewing  a 
solution  against  a  black  background,  in  other  words,  by 
light  coming  chiefly  from  one  side.  How  much  unilateral 
lighting  aids  us  in  recognizing  slight  turbidities  is  familiar  to 
you  from  observing  dust  particles  in  a  ray  of  sunlight. 
When  a  pencil  of  bright  light  is  thrown  into  a  darkened  room 
we  not  only  see  the  bright  cone  but  in  it  a  large  number  of 
dust  particles  which  escaped  us  when  the  light  came  from  all 
sides.  This  method  of  demonstrating  fine  turbidities  was 
used  even  by  FARADAY  to  prove  the  disperse  nature  of 


50  COLLOID  CHEMISTRY 

solutions  of  colloid  gold.  In  this  way  he  demonstrated 
the  disperse  nature  of  red  gold  which  ordinarily  seems 
entirely  clear.  The  method  was  used  in  more  extensive 
fashion  by  J.  TYNDALL,  in  whose  honor  we  call  the  light  cone 
observed  in  dispersoids  when  illuminated  from  one  side  only, 
the  TYNDALL  cone. 

The  majority  of  all  typical  colloids  shows  a  Tyndall  cone. 
Since  this  is  a  matter  of  much  interest  I  shall  demonstrate 
it  to  you  (demonstration). 


FIG.  21.  — A  TYNDALL  cone. 

We  have  here  an  arc  light  from  which  we  obtain  a  narrow 
pencil  of  light  which  passes  into  this  vessel  with  parallel 
sides  filled  with  distilled  water.  You  observe,  I  hope, 
nothing  but  a  slight  glow  in  the  water.  If  the  water  were 
absolutely  pure,  if  it  did  not  contain  even  the  slightest 
traces  of  dust  or  air,  and  were  I  able  to  shut  out  all  reflection 
from  the  walls  of  the  vessel,  you  would  see  nothing  at  all. 
It  is  experimentally  possible  to  produce  water  which  to  the 


CLASSIFICATION  OF  THE  COLLOIDS  51 

naked  eye  is  thus  ''optically  empty."  Let  me  now  pour 
into  this  vessel  a  few  cubic  centimeters  of  a  brown  solution 
of  colloid  silver,  which  as  you  saw  before,  is  also  perfectly 
clear  to  the  naked  eye.  As  the  two  liquids  mix,  an  intense 
greenish- white  cone  of  light  flashes  into  view  (Fig.  21). 
This  is  the  famous  TYNDALL  cone  and  is  due  to  the  optical 
heterogeneity  of  colloid  solutions. 

I  could  take  up  one  after  the  other  of  the  colloid  solutions 
on  the  table  and  hi  almost  every  one  of  them  show  you  this 
TYNDALL  effect. 

There  are,  of  course,  colloids  which  show  it  but  little, 
such  as  blood  serum,1  alkali  albuminates,  freshly  prepared 
silicic  acid  and  very  pure  congo  red  solutions.  These 
colloids  belong  to  that  second  class  of  emulsoids  to  which  I 
have  already  called  your  attention  and  which  we  shall 
discuss  in  detail  in  the  next  lecture.  These  colloids  are 
characterized  by  their  great  hydration  or  salvation.  Their 
particles  have  taken  up  a  large  amount  of  the  dispersion 
media;  in  fact,  they  may  at  times  consist  chiefly  of  this. 
This  explains  why  such  colloids  show  the  TYNDALL  effect 
but  weakly.  To  change  the  direction  of  a  light  ray  it  is 
necessary  that  a  distinct  refraction  difference  exist  between 
dispersed  phase  and  dispersion  medium.  But  if  the  colloid 
particles  are  largely  built  up  of  the  dispersion  medium  itself, 
the  optical  difference  between  combined  and  uncombined 
dispersion  medium  is  but  slight.  This  is  why  typical 
colloids  and  even  coarse  dispersions  do  not  necessarily 
appear  turbid.  Coarsely  dispersed  powdered  glass  in 
Canada  balsam  of  the  same  coefficient  of  refraction,  for 
instance,  does  not  appear  turbid.  One  must  be  careful, 
therefore,  to  avoid  the  common  mistake  of  concluding  that 
a  material  is  highly  dispersed  just  because  it  is  not  turbid.2 

1  According  to  F.  BOTTAZZI  fresh  blood  serum  is  practically  clear  opti- 
cally; see  WINTERSTEIN,  Handbuch  der  vergleichenden  Physiologic,  1,  145. 

2  A  silicic  acid  solution  showing  practically  no  TYNDALL  phenomenon 
has  been  observed  by  C.  O.  WEBER,  Chemistry  of  India  Rubber,  3rd  edition, 
74,  London,  1909. 


52  COLLOID   CHEMISTRY 


To  what  degree  of  dispersion  may  a  material  be  carried 
and  still  show  this  TYNDALL  phenomenon?  And  what 
changes  does  it  show  as  we  pass  over  into  the  field  of  molec- 
ularly  dispersed  solutions?  Even  typical  molecular  disper- 
soids  may,  under  certain  circumstances,  show  the  TYNDALL 
effect.  A  saturated  solution  of  cane  sugar  shows  it 
(demonstration),  but  a  dilute  (one  percent)  solution  does 
not.  The  latter  shows  little  more  than  the  pure  distilled 
water  I  placed  before  you  previously  (demonstration).1 
Not  only  substances  of  high  molecular  weight  like  cane  sugar 
but  also  ordinary  salts  like  the  different  sulphates  show  a 
clearly  marked  TYNDALL  effect,  as  the  Belgian  investigator 
W.  SPRING  has  shown.  Even  when  we  exclude  cases  in 
which  a  colloid  precipitate  may  be  produced  through  hy- 
drolysis, as  in  the  metallic  salts,  there  remains  plenty  of 
evidence  that  solutions  of  substances  of  high  molecular 
weight  and  highly  concentrated  solutions  of  even  simple 
substances  may  exhibit  the  TYNDALL  phenomenon.  Such 
concentrated  solutions  we  know  from  other  evidence  to 
contain  molecular  aggregates,  polymers,  etc.,  so  that  this 
optical  behavior  comes  to  be  in  entire  keeping  with  our 
concept  of  continuity. 

Do  these  phenomena  of  optical  heterogeneity  disappear  as 
we  get  into  the  realm  of  the  molecular  aggregates  and  the 
large  molecules,  or  can  we  still  make  them  out  in  the  realm 
of  the  higher  dispersoids?  To  answer  this  question  correctly, 
we  need  to  bear  in  mind  that,  as  we  ascend  the  dispersion 
scale,  optical  heterogeneity  becomes  attributable  more  and 
more  to  optical  causes  other  than  the  mere  lateral  deviation 
of  light  rays  observed  in  coarsely  dispersed  systems.  While 
in  the  latter  the  turbidness  is  chiefly  attributable  to  refrac- 

1  A  practically  saturated  cane  sugar  solution  is  one  of  the  best  of  ma- 
terials to  use  in  demonstrating  the  TYNDALL  phenomenon.  The  intense, 
uniform  often  bluish,  luminous  cone  which  disappears  upon  dilution  proves 
that  we  do  not  deal  in  this  instance  with  impurities.  See  some  of  the  fol- 
lowing notes. 


CLASSIFICATION  OF  THE  COLLOIDS  53 

tion,  that  in  the  most  highly  dispersed  systems  is  due  to 
diffraction.  When  the  diameter  of  the  dispersed  particles 
falls  below  that  of  the  length  of  the  light  waves  illuminating 
them,  refraction  in  the  ordinary  sense  of  the  word  can  no 
longer  take  place.  Instead,  a  diffuse  dispersion  of  the  light 
in  all  directions  takes  place.  This  occurs  even  in  colloids, 
for  colloid  particles  have  already  a  diameter  of  but  half  a 
wave  length  or  less.  The  TYNDALL  phenomenon  as  ob- 
served in  colloid  solutions  is  therefore  really  due  to  light 
dispersion.  It  follows  from  the  nature  of  this  dispersion 
that  when  mixed  light  is  used  the  shorter  rays  are  bent  mare 
than  the  longer  ones.  The  blue,  violet  and  ultraviolet  rays 
of  a  TYNDALL  cone  will,  therefore,  be  bent  more  than  the 
yellow  and  red  rays.  This  results  in  that  play  of  colors 
known  as  opalescence,  to  which  we  shall  return  in  a  moment. 
Furthermore,  it  is  clear  that  the  short  waves  will  still  be  bent 
by  particles  too  small  to  bend,  for  instance,  the  blue,  yellow 
or  red  rays.  This  is  a  matter  of  much  interest  to  us.  When 
the  TYNDALL  method  is  so  refined  that  we  are  enabled  to 
perceive  not  only  mixed  blue  or  violet  light  but  ultraviolet 
light,  then  it  becomes  possible  to  recognize  a  turbidity  even  in 
molecularly  dispersed  systems.  Just  as  colloid  particles  may 
interfere  with  the  longer  waves  of  visible  light,  molecules 
may  effect  a  disturbance  in  the  shorter  ultraviolet  rays.1 
One  way  of  refining  the  TYNDALL  method  is  to  employ 
photography,  which  you  know  to  be  particularly  effective 
in  proving  the  presence  of  the  chemically  active  ultraviolet 
rays.  I  have  myself  observed  that  distilled  water  which  is 
optically  clear  to  the  naked  eye  shows  a  marked  TYNDALL 
cone  photographically.  But  this  is  lost  on  interposing  be- 
tween water  and  camera  a  thick  glass  plate  which  absorbs 
most  of  the  ultraviolet  rays.  By  employing  ultraviolet 
TYNDALL  cones,  a  number  of  investigators  have  recently 
obtained  results  which  can  be  explained  only  by  saying  that 
turbidities  exist  even  in  typical  molecularly  dispersed 

1  Regarding  TYNDALL  cones  due  to  ultraviolet  and  even  shorter  light 
waves,  see  WOLFGANG  OSTWALD,  Koll.-Zeitschr.,  13,   121   (1913). 


54  .  COLLOID  CHEMISTRY 

systems.1  We  can  work  with  still  shorter  rays  to  demon- 
strate turbidities  of  molecular  dimensions,  by  using,  for 
example,  RONTGEN  rays,  which  are  but  0.04  to  0.06  ^  long 
and  which,  according  to  the  observations  of  C.  BARKLA  and 
others,  can  also  be  deviated  from  their  course.  The  problem 
has  recently  been  solved  in  beautiful  fashion  by  the  investi- 
gations of  M.  LAUE  and  his  co workers  who  succeeded  in 
photographing  the  refraction  pictures  of  RONTGEN  rays 
passed  through  crystals  and  in  this  way  obtained  pictures 
consisting  of  tiny  spots  of  light  arranged  in  a  manner  corre- 
sponding to  the  system  of  the  crystals  studied.  Each  of 
the  spots  upon  the  photographic  plate  was  made  by  a  con- 
centrated pencil  of  RONTGEN  rays  and  showed  an  intensity 
dependent  upon  the  spacial  orientation  of  the  molecules  in 
the  crystal.  The  authors  themselves  hold  that  their  results 
not 'only  prove  the  individuality  of  the  molecule  but  its 
crystalline  and  anisotropic  nature. 

One  can  hardly  imagine  a  more  perfect  and  continuous 
series  of  phenomena  for  proving  the  continuity  of  the 
different  classes  of  dispersed  systems  than  that  represented 
by  the  microscopic  turbidity  of  coarsely  dispersed  systems, 
the  visible  TYNDALL  cone  of  typical  colloids  and  concentrated 
molecular  dispersoids,  the  invisible  ultraviolet  TYNDALL 
cone,  and  finally  the  RONTGEN  ray  TYNDALL  cone  of  sys- 
tems alleged  to  be  homogeneous. 

§10. 

Permit  me  to  return  for  a  moment  to  the  TYNDALL 
phenomenon  as  observed  in  typical  colloids.  It  is  of  much 

1  This  greater  sensitiveness  of  a  TYNDALL  cone  to  photographic  investi- 
gation which  I  have  often  observed  probably  explains  the  discrepancies 
between  the  recent  studies  of  W.  KANGRO  [Zeitschr.  f.  physik.  Chem.,  87, 
257  (1914)]  and  those  of  W.  SPRING.  While  the  latter  succeeded  by  differ- 
ent methods  in  obtaining  water  and  various  salt  solutions  in  a  form  opti- 
cally empty  to  the  human  eye  —  a  possibility  which  every  microscopist  is 
able  to  corroborate  —  the  former  was  able  to  confirm  these  negative  findings 
of  SPRING  only  in  part  when  he  used  photographic  methods.  The  best  method 
for  the  purification  of  molecular  solutions,  namely,  that  of  ultrafiltration, 
was  not  used  by  KANGRO,  a  fact  to  which  attention  has  already  been  called 
elsewhere. 


CLASSIFICATION  OF  THE  COLLOIDS  55 

interest,  for  it  illustrates  a  principle  of  great  importance  in 
the  analytical  methods  of  modern  colloid  chemistry.  As 
you  know,  when  observing  dust  in  the  sun,  we  occasionally 
see  particles  become  visible  temporarily  which  are  so  small 
that  we  miss  them  ordinarily.  If  we  watch  closely,  we  note 
that  these  particles  are  surrounded  by  a  luminous  ring  or 
halo  similar  to  that  observed  along  the  edges  of  an  opaque 
object  when  viewed  against  the  setting  sun.  As  the  object 
viewed  against  the  light  becomes  smaller,  its  edges  become 
contracted  and  the  visual  image  no  longer  corresponds 
accurately  to  the  geometrical  figure.  When  the  particles 
become  sufficiently  small,  the  figure  disappears  entirely  and 
its  place  is  taken  by  a  single  luminous  point.  Similar 
phenomena  are  observed  under  the  microscope  when  we  use 
dark  ground  illumination.  The  arrangement  corresponds  to 
a  viewing  of  the  TYNDALL  phenomenon  against  a  dark  back- 
ground. It  is  important  to  remember  that  particles  will 
show  this  diffraction  phenomenon  even  when  smaller  than 
a  wave  length  of  light.  The  limits  of  ordinary  microscopic 
visibility,  or,  to  put  it  more  accurately,  the  limits  for  obtain- 
ing a  correct  geometrical  picture  are  set,  as  I  told  you,  by 
the  length  of  the  light  waves.  We  can  still,  however,  obtain 
diffraction  pictures  or  diffraction  spots  of  particles  which 
are  smaller  than  the  length  of  a  light  wave. 

This  method  of  demonstrating  optically  the  presence  of 
individual  particles  less  than  a  wave  length  in  diameter,  by 
utilizing  the  principles  of  diffraction,  but  with  sacrifice  of 
the  geometrical  image,  we  call  ultramicroscopy.  Since  col- 
loids are  by  definition  dispersed  systems  in  which  the  dis- 
persed particles  have  a  diameter  of  less  than  a  light  wave, 
they  may  be  rendered  visible  by  using  dark  ground  illumina- 
tion. This  was  accomplished  for  the  first  time  by  the  two 
German  investigators,  H.  SIEDENTOPF  and  R.  ZSIGMONDY,  to 
whom  we  are  also  indebted  for  important  developments  in 
ultramicroscopic  methods.  The  importance  of  ultrami- 
croscopic  methods  in  rendering  visible  the  individual  par- 
ticles in  colloid  systems  and  in  thus  proving  the  gradual 


56 


COLLOID  CHEMISTRY 


transition  of  coarsely  dispersed  particles  to  those  of  colloid 
dimensions  is  self-evident. 

You  obtain  a  good  idea  of  the  principles  of  ultramicroscopy 
if  you  imagine  yourself  looking  at  a  TYNDALL  cone  with  a 
lens  or  a  microscope.  The  Belgian  investigator  W.  SPRING 
many  years  ago  used  a  magnifying  glass  on  TYNDALL  cones 
and  the  beginning  of  our  present  day  ultramicroscopy  may 
be  seen  in  the  arrangements  for  dark  ground  illumination  so 


FIG.  22.  —  A  simple  arrangement  for  ultramicroscopy  according  to 
R.  ZSIGMONDY. 

long  used  by  bacteriologists  and  students  of  diatoms.  We 
can,  of  course,  use  high  powers  of  the  microscope  to  accom- 
plish the  optical  analysis  of  a  TYNDALL  cone.  The  brightest 
spot  of  a  small  TYNDALL  cone  is  then  thrown  just  below  the 
objective  of  a  microscope,  as  shown  in  Fig.  22.  It  would 
take  us  too  far  afield  were  I  to  discuss  the  details  of  construc- 
tion of  ultramicroscopes  or  to  tell  you  of  the  many  observa- 
tions that  have  been  made  with  them.  I  shall  only  point 
out  that  such  suspensoids  as  the  colloid  solutions  of  the 
metals  yield  varied  and  often  extraordinarily  colored  ultra- 
microscopic  pictures.  The  TYNDALL  cone  produced  by  a 
solution  of  colloid  gold  is  filled  with  innumerable  brilliant 


CLASSIFICATION  OF  THE  COLLOIDS  57 

points  showing  beautiful  BROWNIAN  movement.  On  the 
other  hand,  such  things  as  proteins  and  certain  dyes  do  not 
analyze  into  "ultramicrons."  This  is  not  always  because 
the  particles  are  too  small  but  because,  as  previously  stated, 
they  are  so  highly  hydrated  that  their  coefficient  of  refrac- 
tion is  not  very  different  from  that  of  their  dispersion  media.1 
These  facts  can  be  better  demonstrated  than  described  and 
so  I  must  ask  you  to  wait  for  the  experiment  which  I  shall 
present  at  the  end  of  this  lecture.2  What,  now,  are  the 
limits  of  visibility  of  particles  ultramicroscopically?  This 
question  is  of  interest  in  connection  with  our  concept  of 
continuity.  Suffice  it  to  say  that  such  limits  are  in  high 
degree  dependent  upon  the  intensity  of  the  illumination. 
By  using  light  from  an  arc  or  from  the  sun  we  may  still 
establish  the  existence  of  particles  having  a  diameter  of  a 
few  millimicrons.  Of  course  the  photochemical  effects  of 
the  light  are  often  so  intense  in  such  investigations  that  they 
are  carried  out  with  great  difficulty. 

In  connection  with  our  concept  of  continuity,  it  is  a  matter 
of  importance  that  R.  ZSIGMONDY  was  able  to  produce  rose 
colored  colloids  of  gold  which  were  so  highly  dispersed  that 
they  could  not  be  analyzed  under  the  ultramicroscope  even 
when  direct  sunlight  was  used,  a  behavior  in  keeping  with 
the  previously  discussed  fact  that  these  colloids  show  also  a 
distinct  tendency  to  diffuse. 

§11. 

Of  much  interest  is  the  color  of  colloid  systems.  One  of 
the  simplest  of  the  questions  under  this  heading  is  that  of 
their  opalescence,  or,  differently  expressed,  the  "  color  of  the 

1  This  mistake  of  concluding  from  negative  ultramicroscopic  findings  that 
the  material  in  hand  is  therefore  necessarily  a  molecularly  dispersed  one  is 
still  made. 

2  I  was  in  the  habit  of  demonstrating  at  the  end  of  this  lecture  such 
ultramicroscopic   apparatus  as  circumstance  provided,   the  nature  of  the 
demonstration  being  determined  by  the  number  in  the  audience  and  its 
interest.     For  a  detailed  description  of  ultramicroscopic  pictures,  the  reader 
is  referred  to  the  text-books  of  colloid  chemistry. 


58  COLLOID  CHEMISTRY 

colorless  colloids."  All  sorts  of  solid,  liquid  and  gaseous 
colloids  are  bluish  or  violet  when  viewed  against  a  dark 
background;  or  yellow  and  red  when  we  look  through  them. 
You  need  but  look  at  these  solutions  of  gelatin  or  albumin, 
at  this  colloid  mastic  (prepared  by  pouring  an  alcoholic 
solution  of  mastic  into  water),  at  this  milk  glass,  and  at  this 
white  opal1  (demonstration).  The  greatest  example  of  a 
gaseous  dispersoid  exhibiting  opalescence  is  seen  in  the  sky. 
When  we  look  at  a  cloudless  sky  against  the  dark  back- 
ground of  space,  this  heavenly  dispersoid  looks  blue;  but 
when  we  look  through  it  against  a  source  of  light  (as  against 
the  sun  when  it  is  coming  up  or  going  down)  we  find  it  yellow 
or  red.  The  cause  of  this  opalescence  is  to  be  found  in  the 
fact  that  the  longer  yellow  and  red  rays  are  less  disturbed 
and  bent  in  a  highly  dispersed  system  than  are  the  shorter 
violet  and  blue  rays. 

To  show  you  that  not  only  yellow  and  blue  colors  may 
appear  in  dispersed  systems  composed  of  two  substances  in 
themselves  colorless,  I  present  this  flask  of  polymerized 
cinnamic  ethyl  ester.  You  note  a  beautiful  greenish-red 
opalescence  which  would  give  way  to  a  bluish-yellow  were 
I  to  warm  the  mixture.  The  system  consists,  as  shown  on 
TYNDALL  analysis,  of  a  mixture  of  monomolecular  ester  and 
polymerized  ester  of  which  the  particles  have  attained  at 
least  colloid  dimensions.  You  may  note  similar  color 
phenomena  in  the  gelatinous  colloid  sodium  chlorid  pre- 
viously shown  you.  In  fact,  you  may  observe  such 
CHRISTIANSEN  opalescence  when  you  merely  powder  sodium 
chlorid  very  finely  and  suspend  it  in  a  mixture  of  benzol 
and  carbon  disulphid  of  practically  the  same  coefficient  of 
refraction  as  the  sodium  chlorid  itself.  It  would  take  us  too 
far  afield  to  enter  into  the  theory  of  these  interesting 
phenomena.2 

1  The  play  of  colors  in  the  opal  is  due  in  part  only  to  opalescence,  in  part 
to  the  interference  colors  produced  by  thin  plates. 

2  A  detailed  discussion  of  these  CHRISTIANSEN  colors  and  of  related  prob- 
lems will  be  found  in  a  volume  entitled,  Light  and  Color  in  Colloids,  which 
I  hope  to  be  able  to  publish  soon. 


CLASSIFICATION  OF  THE  COLLOIDS 


59 


Opalescence  also  varies  greatly  with  the  degree  of  dis- 
persion. Coarsely  dispersed  systems  are  but  slightly  opal- 
escent, while  colloid  systems  are  intensely  so.  So  far  as 
the  Opalescence  of  molecular  dispersoids  is  concerned,  we 
find  that  some  investigators,  like  LORD  RAYLEIGH,  assume 
that  opalescence  may  still  be  produced  through  the  dis- 
turbing effects  of  molecules.  This  is  the  case,  for  example, 
in  certain  gases,  and  it  is  held  that  at  least  a  part  of  the 
opalescence  of  the  sky  may  be  dependent  upon  such  a 
selective  bending  of  light  rays  by  the  molecules  of  the 

DISPERSED  SYSTEMS 
COARSE  DISPERSIONS-COLLOIDS-MOLECULAR  DISPERSOIDS 

(lOOtol/x/O 

Increasing  degree  of  dispersion »~ 


Maximum 

In  the  colloid  realm. 

Opalescence,  Intensity  of  color, 

Catalytic  activity,  etc. 


FIG.  23. 

atmosphere.  Even  when  we  grant  this  molecular  opales- 
cence, the  fact  remains  that  its  intensity  decreases  as  we 
pass  from  the  colloid  into  the  molecular  region.  This  is 
amply  illustrated  by  the  fact  that  most  salt  solutions  and 
most  gases  are  colorless  unless  viewed  in  enormous  thick- 
nesses, as  presented  by  great  masses  of  water  or  in  the 
earth's  atmosphere. 

We  note  in  this  discussion  a  first  illustration  of  the  fact 
that  the  intensity  of  a  property  of  dispersed  systems  may 
show  a  maximum,  and  this  in  the  colloid  realm.  Previously 
we  have  only  dealt  with  properties  which  either  increased 
steadily  with  increase  in  degree  of  dispersion  (as  BROWNIAN 
movement  and  diffusion  velocity)  or  such  as  decreased 
steadily  (as  the  phenomena  of  heterogeneity)  as  we  passed 


60 


COLLOID  CHEMISTRY 


from  coarse  dispersions  through  colloids  to  molecular  dis- 
persions. As  we  proceed,  we  shall  find  further  illustrations 
of  how  certain  properties  of  dispersoids  may  show  a  maxi- 
mum or  minimum  in  the  colloid  realm,  as  indicated  in 
Fig.  23. 

§12. 

The  next  property  to  be  discussed,  that  of  the  intensity 
of  color  in  colloid  solutions,  illustrates  this.  As  is  well 
known,  such  colloid  salts  of  the  metals  as  the  sulphids  may 
show  so  marked  a  color  even  hi  very  low  concentrations 


o 

!• 


1                 2               3               4              56  1 

Degree  of  dispersion ». 

FIG.  24.  —  Relation  of  color  intensity  of  colloid  gold  to  its  degree  of 
dispersion  according  to  THE  SVEDBERG. 

that  it  may  be  used  for  their  qualitative  recognition.  The 
coloration  intensity  of  these  colloids  may  at  times  be  greater 
even  than  that  of  the  aniline  dyes.  Thus  if  the  coloring 
intensity  of  fuchsin  is  represented  by  the  arbitrary  value 
of  5,  that  of  colloid  iron  hydroxid  is  about  the  same,  while 
that  of  arsenic  trisulphid  is  100,  and  that  of  colloid  gold 
about  2000  (THE  SVEDBERG).  When  the  coloration  inten- 
sity of  a  substance  in  different  degrees  of  dispersion  is 
studied,  it  is  found  to  attain  a  maximum  in  the  realm  of 
colloid  dispersion. 


CLASSIFICATION  OF  THE  COLLOIDS  61 

If  we  choose  gold  as  an  example,  it  is  easily  seen  that  a 
black,  coarsely  dispersed  gold  precipitate  has  less  covering 
power  than  a  solution  of  gold  containing  the  same  amount 
of  metal  in  colloid  form.  When  colloid  gold  is  coagulated 
or  precipitated,  the  solution  becomes  "decolorized."  The 
small  amount  of  black  precipitate  which  falls  to  the  bottom 
of  the  flask  has  again  a  minimal  covering  power.  You 
will  perhaps  recall  that  in  preparing  colloid  gold  I  started 
with  a  gold  chlorid  solution  which  was  practically  color- 
less; hi  other  words,  one  which  hardly  showed  the  yellow- 
ish-brown color  of  the  ion.  From  this  colorless  solution 
we  obtained  the  intensely  red  and  blue  gold  colloids.  It  is 
therefore  certain  that  colloid  gold  has  a  more  intense  color 
than  either  coarsely  or  molecularly  or  ionically  dispersed 
gold;  in  other  words,  a  maximum  is  observed  in  the  colloid 
realm.  The  accompanying  Fig.  24,  taken  from  THE  SVED- 
BERG,  illustrates  quantitatively  the  appearance  of  this  color 
maximum  in  the  colloid  region. 

§13. 

Not  only  is  the  intensity  of  colloid  colors  a  noticeable 
fact,  but  their  beauty  and  variety  as  well.  I  have  already 
shown  you  red  and  blue  gold,  and  by  precipitating  this 
metal  with  oxalic  acid  we  can  obtain  green  gold.  Silver 
and  platinum  in  the  colloid  state  also  show  many  different 
colors.  Gold,  silver  and  platinum  may  therefore  be  re- 
garded as  panchromatic. 

I  have  here  a  number  of  photographic  plates  prepared  by 
LUPPO-CRAMER'S  methods,  which  show  differently  colored 
silver  colloids  in  gelatin  (demonstration).1  These  were,  as 
a  matter  of  fact,  made  by  his  own  hands.  You  observe 
that  the  plates  are  yellow,  orange,  red,  violet  and  blue; 
and  here  I  show  you  one  that  is  green.  When  you  first 
look  at  this  plate  it  is  greenish  violet,  but  I  need  only  dip 
it  in  water  for  a  few  seconds  for  you  to  see  this  color  give 

1  These  preparations  were  kindly  given  me  by  Drs.  LUPPO-CRAMER  and 
R.  E.  LIESEGANG.  I  should  like  here  also  to  thank  them  for  their  kindness. 


62  COLLOID  CHEMISTRY 

way  to  a  clear  dark  green  (demonstration).1  All  these 
colors  are  the  colors  of  colloid  silver  and  one  naturally  asks 
why  they  differ  so.  Let  me  point  out  that  we  are  again 
indebted  to  an  American  for  a  first  study  of  this  question. 
CAREY  LEA  recognized  and  investigated  the  colors  of  colloid 
silver  and  gold  many  years  ago. 

The  different  colors  of  one  and  the  same  metal  in  a  col- 
loid state  are  chiefly  referable  to  differences  in  their  degree 
of  dispersion.  There  is,  of  course,  much  room  for  differ- 
ences in  degree  of  dispersion  even  within  the  realm  of  the 
colloid  dimensions  themselves  which  lie  between  100  and 
1  /I/*,  and  it  is  in  these  finer  differences  that  the  explana- 
tion of  the  color  changes  must  be  sought.  To  prove  this, 
I  need  but  add  some  hydrochloric  acid  to  this  red  colloid 
gold,  and  stir  the  mixture  (demonstration).  The  acid  coag- 
ulates the  gold;  in  other  words,  it  changes  the  gold  to  a 
coarsely  dispersed  precipitate.  A  first  effect  is  a  change  in 
color.  As  you  see,  the  mixture  becomes  violet.  In  a  little 
while  it  will  become  greyish  black.2  The  different  colors 
of  silver  which  I  showed  you  are  also  to  be  explained  through 
such  differences  in  the  sizes  of  the  metallic  particles,  de- 
pendent upon  the  methods  used  in  their  preparation. 

The  order  in  which  the  colors  change  from  one  to  the 
other  as  the  degree  of  dispersion  changes  seems  also  to  be 
definite.  As  a  rule,  the  most  highly  dispersed  colloid 
metals  are  yellow  or  orange;  in  other  words,  they  absorb 
violet  and  blue  light.  As  the  degree  of  dispersion  de- 
creases, the  color  passes  from  yellow  through  orange  to 
red,  violet,  blue  and  finally  green.  The  absorption  maxi- 
mum gradually  moves  toward  the  side  of  the  greater  wave 
lengths  as  the  degree  of  dispersion  decreases.3  The  same 

1  See  Lftppo-CRAMER,  Koll.-Zeitschr.,  8,  240  (1911). 

2  These  experiments  are  best  made  with  large  quantities  of  the  red  colloid 
gold  contained  in  two  cylinders.     Not  too  much  of  the  hydrochloric  acid 
must  be  added  at  once,  otherwise  precipitation  in  coarse  form  occurs  at 
once  without  the  change  to  the  blue  color  being  clearly  visible. 

3  For  further  details  regarding  this  relation  between  color  and  degree  of 
dispersion,  see  WOLFGANG  OSTWALD,  Kolloidchem.  Bern.,  2,  409  (1911),  as 
well  as  my  forthcoming  monograph,  Light  and  Color  in  Colloids. 


CLASSIFICATION  OF  THE  COLLOIDS 


63 


order  is  frequently  observed  in  organic  dyestuffs  when 
the  colors  of  any  homologous  series  are  studied.  Yellow 
is  usually  the  color  of  the  chemically  simpler  members, 
while  the  dyes  of  greater  molecular  complexity  in  the  same 
series  are  often  blue  and  violet. 

What  about  transition  phenomena  to  be  observed  in  pass- 
ing from  the  colloids  either  in  the  direction  of  the  coarse 
dispersions  or  in  that  of  the  molecular  systems?  Follow- 
ing our  definition  of  colloids  we  are  not  only  justified  but 


1000 


500 


400 


500 


700 


FIG.  25. —  Variation  in  light  absorption  by  colloid  gold  with  change  in  its 
degree  of  dispersion  according  to  THE  SVEDBERG.  The  curve  farthest  to 
the  right  is  that  of  the  coarsest  colloid.  The  curve  farthest  to  the  left  is 
that  of  a  molecularly  dispersed  gold  colloid  solution. 

in  duty  bound  to  ask  this  question.  I  have  already  showed 
you  how  the  colors  of  such  colloid  metals  as  gold  change 
from  red  to  violet  and  blue  as  the  degree  of  dispersion  de- 
creases. But  what  is  the  color  of  less  colloid  or  coarsely 
dispersed  gold?  Thin  leaves  of  gold  have  in  transmitted 
light  a  distinctly  greenish  color,  and  I  have  told  you  that 
gold  may  be  obtained  as  a  greenish  precipitate.  But  what 
is  the  nature  of  the  transition  phenomena  as  we  pass  from 
the  colloid  to  the  molecularly  dispersed  or  ionically  dis- 


64  COLLOID  CHEMISTRY 

persed  gold?  The  most  highly  dispersed  colloid  gold  thus 
far  prepared  is  ruby-red  or  yellowish-red.1  Ionic  gold  in 
the  presence  of  a  colorless  anion  is  distinctly  brownish- 
yellow  or  orange.  An  accurate  study  of  this  transition  can- 
not be  made  with  the  naked  eye,  but  may  be  made  spectro- 
scopically.  In  this  way  THE  SVEDBERG  has  shown  that 
the  absorption  curves  of  colloid  gold  gradually  approximate 
the  absorption  curves  of  ionic  gold  as  the  degree  of  colloid 
dispersion  increases.2  The  curves  of  Fig.  25  illustrate  this. 
The  lowermost  curve  is  the  absorption  curve  of  molecular 
gold  chlorid.  The  uppermost  one  is  that  of  a  relatively 
coarse  colloid  gold.  The  curves  between  are  those  of 
colloids  of  successively  greater  degrees  of  dispersion. 

Let  us  consider  next  the  colors  of  colloid  silver.  Here 
also  the  color  passes  from  yellow  through  orange,  red,  vio- 
let and  blue  to  green  in  the  coarsest  members  of  the  series. 
But  according  to  G.  QUINCKE  and  others,  the  color  of  thin 
silver  foil  is  also  blue  or  green.  The  transition  from  the 
colloid  realm  to  that  of  the  coarse  dispersions  is  therefore 
perfectly  smooth.  On  the  other  hand,  the  most  highly 
dispersed  silver  colloids  are  a  transparent  yellow  or  greenish 
yellow;  in  other  words,  they  absorb  chiefly  violet  and 
ultraviolet  light.  The  more  highly  dispersed,  the  greater 
is  the  transparency  of  these  silver  colloids.  When  prepared 
from  very  dilute  solutions,  or,  differently  expressed,  under 
conditions  which  yield  colloids  of  particularly  high  degrees 
of  dispersion,  the  yellow  of  these  colloids  becomes  so  faint 
as  to  be  hardly  recognizable.  But  this  gradual  disappear- 
ance of  color  in  a  highly  dispersed  colloid  silver  marks  the 
passage  from  the  color  of  the  colloid  into  that  of  the  silver 
ion.  The  latter,  in  the  presence  of  a  colorless  anion,  is 
colorless. 

Similar  considerations  hold  for  the  colors  of  colloid  plati- 
num. The  most  highly  dispersed  colloid  platinum  known, 

1  See  THE  SVEDBERG,  Koll.-Zeitschr.,  4,  168  (1909);  5,  318  (1909). 

2  See  THE  SVEDBERG,  I.e.,  as  well  as  numerous  other  papers  appearing 
in  the  Kolloid-Zeitschrift  and  the  Zeitschrift  fur  physikalische  Chemie. 


CLASSIFICATION  OF  THE  COLLOIDS 


65 


that  prepared  by  L.  WOHLER,  is  orange  red,  while  the  color 
of  platinum  salts  is  orange  yellow. 

What  has  been  said  above  holds  not  only  for  the  colors 
of  colloid  metals  but  for  those  of  the  organic  colloid*  dyes 
as  well.  I  show  you  in  Fig.  26  the  absorption  curves  of 
indigo  of  different  degrees  of  dispersion  (THE  SVEDBERG). 
In  colloid  form  in  aqueous  solution  indigo  is  blue,  but, 


i  A 


20000- 


10000 


400  500  600  700 

FIG.  26. — Absorption  of  light  by  indigo  solutions  of  different  degrees 
of  dispersion. 

when  molecularly  dissolved,  as  in  hot  petroleum  or  chloro- 
form, it  is  red  to  violet.  The  lowermost  curve  (2)  shows 
the  absorption  spectrum  of  an  old  and  therefore  relatively 
coarsely  dispersed  colloid;  curve  1A,  the  absorption  curve 
of  an  ordinary  colloid;  curve  B,  that  of  a  molecular  indigo 
in  chloroform.  You  observe  how  the  curves  lie  progres- 
sively higher  as  the  degree  of  dispersion  increases,  while 
at  the  same  time  they  move  from  the  side  of  the  shorter 
wave  lengths  to  that  of  the  longer.1 

1  In  the  experiments  represented  in  the  figure,  the  concentration  of  the 
colloids  is  only  about  half  that  of  the  molecular  solution.  The  specific  color 
intensity  may  not  therefore  be  deduced  from  the  figure.  See  THE  SVEDBERG, 
Die  Existenz  der  Molekule,  51. 


66 


COLLOID  CHEMISTRY 


§14. 

The  principle  of  continuity  holds  not  only  for  the  in- 
tensity and  quality  of  the  colors  of  dispersed  systems  but 
also  for  their  optical  rotation.  Fig.  27  shows  a  series  of 
curves  which  E.  NAVASSART  and  1 2  obtained  in  a  study 
of  the  optical  rotation  of  tannin  of  different  degrees  of  dis- 
persion. The  ordinary  aqueous  solution  of  tannin,  made 
by  dissolving  tannin  in  water,  represents  a  polydispersoid, 
in  other  words,  one  in  which  there  are  particles  of  different 
degrees  of  dispersion.  Most  of  the  tannin  is  in  colloid  solu- 

70 


60 


10 


:Dialyzed  through  parchment  paper 
Normal  in  alcohol 


1       2 


5       6       78 
Concentration 


9      10     11     l£]f 


FIG.  27.  —  Influence  of  degree  of  dispersion  upon  the  optical  rotation 

of  tannin. 

tion  yet  some  of  it,  as  even  GRAHAM  knew,  will  pass  through 
parchment  paper,  and  is  therefore  in  a  state  of  higher 
dispersion.  By  using  different  grades  of  dialyzing  mem- 
branes we  can  obtain  different  fractions  of  the  aqueous 
tannin.  Tannin  is  molecularly  soluble  in  organic  solvents. 
When  the  optical  rotations  of  these  different  tannin  solu- 
tions are  compared,  the  coarsest  tannin  is  found  to  produce 
the  greatest  rotation,  the  molecularly  dispersed  tannin  the 

2  E.  NAVASSART,  Koll.-Zeitschr.,  12,  97  (1913);    Kolloidchem.  Beih.,  6, 
299  (1914). 


CLASSIFICATION  OF  THE  COLLOIDS  67 

least.  The  behavior  of  two  such  tannin  solutions,  repre- 
senting the  extremes  of  dispersion,  is  indicated  in  the  upper- 
most and  lowermost  curves  of  Fig.  27.  Between  them 
are  found  the  curves  characteristic  of  tannin  solutions 
which  have  dialyzed  through  parchment  paper  and  fish 
bladder.  These  membranes  have  different  sized  pores,  the 
fish  bladder  allowing  larger  aggregates  to  pass  through  than 
the  parchment  paper.  As  the  order  of  the  curves  shows 
clearly,  the  specific  optical  rotation  increases  progressively 
in  passing  from  the  molecularly  dispersed  tannin  to  its 
colloid  form. 

I  believe  you  will  agree  with  me  when  I  say  that  a  study 
of  the  changes  in  optical  properties  accompanying  changes 
in  the  degree  of  dispersion  proves  in  unequivocal  fashion 
the  correctness  of  the  principle  of  continuity. 

§15. 

Not  all  the  physical  properties  of  dispersed  systems  have 
as  yet  been  studied  systematically  from  this  point  of  view. 
But  this  is  hardly  to  be  wondered  at,  for  this  way  of  look- 
ing at  the  question  is  not  yet  ten  years  old.1  We  strike  a 
difficult  and  still  ill-defined  field  in  the  general  relations 
between  degree  of  dispersion  and  the  electrical  behavior  of 
colloids.  Since  it  covers  a  series  of  properties  which  play 
a  great  role  in  special  colloid  chemistry  I  shall  touch  upon 
it  briefly. 

Most  colloid  systems,  like  most  coarse  dispersions,  have 
an  electric  charge.  We  recognize  it  and  its  sense  by  send- 
ing an  electric  current  through  the  system.  When  electri- 
cally charged,  a  colloid  moves  in  an  electric  field  —  we 
observe  the  phenomenon  of  electrophoresis.  The  migra- 

1  WOLFGANG  OSTWALD,  Koll.-Zeitschr.,  1,  291,  331  (1907);  also  Grundriss 
der  Kolloidchemie,  1.  Aufl.  Dresden,  1909.  Here  was  emphasized,  so  far  as 
I  know  for  the  first  time,  the  importance  of  the  study  of  these  transition 
phenomena  and  here  too  was  suggested  the  curve-like  nature  of  the  vari- 
ations in  physico-chemical  properties  with  changes  in  degree  of  dispersion. 
Two  years  later  appeared  the  observations  of  THE  SVEDBERG  and  his  pupils 
which  followed  the  suggestions  of  my  earlier  papers. 


68  COLLOID  CHEMISTRY 

tion  of  a  colloid  is  easily  seen  when  the  points  of  two  wire 
electrodes  are  dipped  into  a  drop  of  any  dispersoid  on  a 
microscopic  slide,  or  when  the  dispersoid  is  poured  into  a 
U  tube  and  a  current  is  sent  through  it.  I  show  you  here 
two  U  tubes  in  one  of  which  I  have  a  colloid  gold,  in  the 
other  a  colloid  iron  hydroxid  (demonstration).1  An  elec- 
tric current  from  a  small  storage  battery  has  been  passing 
for  some  hours  through  both.2  The  tubes  have  been  placed 
in  parallel.  To  the  right  is  the  positive  pole,  the  anode. 
You  observe  how  in  the  tube  containing  colloid  gold  the 
liquid  is  almost  colorless  about  the  cathode,  while  it  is  a 
dark  violet  color  about  the  anode.  Colloid  gold  wanders 
to  the  positive  pole.  It  is  therefore  negatively  charged. 
A  reverse  behavior  is  observed  in  the  iron  hydroxid.  The 
colloid  has  collected  in  thick  flakes  about  the  cathode. 
It  is  therefore  positively  charged. 

It  is  often  possible  to  detect  the  sense  of  the  electric 
charge  of  a  colloid,  even  without  the  aid  of  a  current,  by 
very  simple  means.  I  have  hung  up  here  some  strips  of 
ordinary  filter  paper,  the  lower  ends  of  which  may  be 
dipped  into  different  colloid  solutions.  As  you  know, 
liquids  tend  to  ascend  such  strips  of  filter  paper  through 
capillarity.  I  have  here  the  colloid  solutions  of  two  blue 
dyes,  night  blue  and  alkali  blue.  Let  me  dip  the  ends  of 
two  filter  paper  strips  into  them,  and  even  as  I  talk  to  you, 
you  note  the  following:  in  the  night  blue  the  dye  ascends 
together  with  the  aqueous  dispersion  medium.  The  water 
is  slightly  ahead,  but  nevertheless  the  dye  follows  close 
behind.  In  fifteen  minutes  it  may  have  covered  ten  or 
more  centimeters  (demonstration).  The  alkali  blue  behaves 
totally  differently.  The  water  ascends  far  in  advance  of 
the  dye.  In  other  words,  there  is  a  separation  of  the  dye 

1  Instead  of  these  colloids,  Berlin  blue  or  colloid  graphite  (negative)  or 
night  blue  and  alkali  blue  (positive  and  negative)  may  be  used. 

2  Since  the  speed  of  electrophoresis  is  roughly  proportional  to  the  differ- 
ence in  potential,  it  is  well  to  use  currents  of  high  potential  and  of  small 
amount.     Overheating  and  the  disturbances  incident  thereto  must  be  care- 
fully avoided. 


CLASSIFICATION  OF  THE  COLLOIDS  69 

from  the  dispersion  medium.  After  a  quarter  of  an  hour 
the  dye  will  have  concentrated  at  a  point  a  little  above 
the  surface  of  the  liquid,  but  it  will  not  have  followed  the 
water  (demonstration). 

Let  me  next  show  you  two  experiments  with  different 
colloids  of  graphite  (demonstration).  The  dispersed  phase, 
the  graphite,  is  the  same  in  both,  but  the  dispersion  media 
are  different.  The  two  have  been  subjected  to  a  " capillary 
analysis."  This  first  dish  contains  colloid  graphite  hi 
water  (aquadag);  the  second,  colloid  graphite  hi  mineral 
oil  (oildag)  diluted  somewhat  with  ligroin.  The  watery 
colloid  has  ascended  with  the  water,  but  in  the  oily  colloid 
the  colloid  phase  has  concentrated  below,  while  the  dis^ 
persion  medium  has  alone  ascended  the  strip. 

According  to  F.  FIGHTER  and  N.  SAHLBOM  this  difference 
in  ascent  is  to  be  explained  through  the  difference  in  the 
electric  charge  of  the  colloids.  Negatively  charged  colloids 
ascend  with  their  dispersion  media,  while  the  positively 
charged  are  held  fast  near  the  surface  of  the  liquid  and 
therefore  separate  from  their  dispersion  media.  The  ex- 
planation of  this  is  to  be  found  hi  the  fact  that  filter  paper 
in  contact  with  water  assumes  a  negative  electrical  charge. 
Therefore  when  a  positively  charged  colloid  comes  in  con- 
tact with  the  paper  the  colloid  becomes  fixed  electrostati- 
cally. A  negatively  charged  colloid,  on  the  other  hand, 
because  of  the  similarity  of  the  charges,  goes  by  undisturbed.1 

An  important  and  much  overlooked  fact  regarding  this 
electric  charge  of  one  and  the  same  dispersed  phase  is  its 
variability.  As  a  rule,  colloid  metals  and  sulphids,  for 
instance,  show  a  negative  electrical  charge,  especially  in 

1  The  experiment  with  colloid  graphite  in  ligroin  is  not  free  from  objec- 
tions. It  is  possible  that  the  filter  paper  assumes  toward  ligroin  and  like 
substances  a  different  charge  than  toward  water.  To  explain  the  behavior 
under  such  circumstances,  the  second  assumption  would  have  to  be  made 
that  the  graphite  in  oil  maintains  its  negative  charge,  which  according  to 
the  experiments  of  G.  QUINCKE  is  not  true  for  the  charge  of  sulphur  particles 
in  turpentine  and  in  water  [Wiedemann's  Annalen,  113,  513  (1861)].  A 
careful  study  of  this  whole  problem  is  much  needed. 


70  COLLOID  CHEMISTRY 

aqueous  dispersion  media.  We  are,  however,  familiar  with 
positively  charged  gold  colloids.  To  give  you  but  one 
striking  illustration  of  the  variability  of  the  electric  charge 
in  one  and  the  same  colloid,  let  me  call  your  attention  to 
the  following  experiment  of  A.  LOTTERMOSER.  Either  a 
positively  or  a  negatively  charged  silver  iodid  may  be 
obtained  at  desire  by  mixing  a  dilute  solution  of  silver 
nitrate  with  one  of  potassium  iodid.  When  the  potassium 
iodid  is  poured  into  an  excess  of  silver  nitrate,  we  obtain 
a  positively  charged  colloid.  If  we  proceed  in  a  reverse 
way,  pouring  the  nitrate  into  the  iodid  so  that  the  latter 
is  present  in  excess,  we  produce  a  negatively  charged  colloid. 

Then  there  are  colloids  which  have  only  a  very  faint 
electrical  charge.  Protein  and  starch  solutions  very  free  of 
electrolytes  belong  in  this  class.  But,  through  the  addition 
of  aluminium  sulphate,  colloid  gold  can  also  be  "  dis- 
charged," or  " oppositely"  charged,  so  that  it  either  may 
not  move  at  all  in  an  electric  field  or  may  move  toward 
the  negative  pole. 

The  velocity  of  colloid  migration  practically  equals  that 
of  ions  and  coarsely  dispersed  particles.  But  since  accurate 
studies  are  still  lacking  on  this  relation  of  degree  of  dis- 
persion to  speed  of  migration  in  an  electric  field,  it  may 
not  yet  be  concluded  that  the  latter  is  independent  of  the 
former.1  Even  among  the  molecular  dispersoids  the  larger 
and  more  complex  ions  migrate  more  slowly  than  those 
of  higher  dispersion.  It  is  reasonable  to  expect,  therefore, 
that  colloids  will  move  still  more  slowly. 

When  we  inquire  about  transition  phenomena,  the  dearth 
of  available  systematic  investigations  and  the  complexity  of 
the  phenomena  make  it  impossible  to  establish  here  as  simple 
and  definite  relations  as  were  possible  when  discussing 
optical  properties.  Even  the  newer  literature  still  argues 
the  question  of  whether  colloids  are  "ions"  or  not.  In 
answering  this,  much  depends  on  what  we  understand  by 
"ions."  If  the  term  is  used  simply  to  cover  all  material 

1  See  for  example  H.  FREUNDLICH,  Kapillarchemie,  233,  Leipzig,  1909. 


CLASSIFICATION  OF  THE  COLLOIDS  71 

carriers  of  electricity,  then  colloids  and  even  coarsely  dis- 
persed particles  must  be  regarded  as  ions,  for  masses  of  elec- 
tricity are,  of  course,  transported  whenever  any  of  these 
dispersed  particles  move  in  an  electric  field.  But  if  by  ions 
we  mean  only  those  material  carriers  of  electricity  which 
follow  the  laws  of  FARADAY,  as  do  the  ions  in  salt  solutions, 
if,  in  other  words,  the  particles  must  always  be  the  carriers 
of  equivalent  amounts  of  electricity,  then  colloid  and 
coarsely  dispersed  particles  are  not  to  be  counted  among 
them;  for  to  the  present  time  no  one  has  proved  the  validity 
of  FARADAY'S  law  for  electrophoresis;  present  knowledge 
really  indicates  that  the  law  does  not  hold  in  its  ordinary 
form  for  colloids.1  The  influence  of  concentration  upon 
electrophone  " conductivity"  seems  to  be  quite  different  in 
the  case  of  the  colloids  and  coarsely  dispersed  systems  from 
that  of  a  similar  change  in  the  case  of  the  molecularly  dis- 
persed electrolytes.  There  exist,  however,  some  interesting 
analogies  between  the  behavior  of  colloids  and  the  properties 
of  gaseous  ions,  in  other  words,  those  responsible  for  electrical 
conductivity  in  gases.2  Among  these  are  such  as  do  not 
follow  FARADAY'S  law.  Here  the  influence  of  changes  in 
concentration  upon  the  conductivity  of  the  gas  is  not  the 
same  as  that  observed  in  aqueous  electrolytes.  Just  as  in 
the  case  of  colloid  electrophoresis,  we  know  gaseous  systems 
in  which  predominate  carriers  of  one  kind,  either  positive 
or  negative  ions.  In  them  electricity  is  carried  predomi- 
nantly in  one  direction.  The  migration  of  gaseous  ions  under 
the  influence  of  an  electric  current  may  be  deviated  by 
exposure  to  a  magnetic  field.  This  is  a  phenomenon  which 
may  be  observed  in  colloids,3  but  not  in  normal  aqueous 

1  For  further  details  regarding  discrepancies  between  the  behavior  of 
normal  and  of  colloid  ions,  see,  for  example,  WOLFGANG  OSTWALD,  Koll.- 
Zeitschr.,  7,  132  (1910).     The  mathematics  on  page  152  of  this  article  re- 
garding the  similarities  in  the  diffusion  coefficients  is,  however,  incorrect, 
since  in  the  definition  of  diffusion  coefficients  an  error  was  made  in  their 
values. 

2  See  the  preceding  footnote. 

3  See  J.  J.  KOSSONOGOW,  Koll.-Zeitschr.,  7,   129  (1910),  as  well  as  my 
remarks  following  KOSSONOGOW 's  paper. 


72  COLLOID  CHEMISTRY 

electrolytes.  These  considerations  would  seem  to  indicate 
that  further  investigation  of  the  analogies  between  electro- 
phone phenomena  as  observed  in  colloids  and  gases  is  likely 
to  yield  simpler  and  better  material  for  the  establishment  of 
our  principle  of  continuity  than  has  thus  far  the  study  of 
the  analogies  between  colloids  and  aqueous  electrolytes. 

§16. 

Allow  me  in  conclusion  to  touch  upon  a  series  of  properties 
which  again  show  the  importance  of  the  degree  of  dispersion 
upon  the  physico-chemical  behavior  of  dispersed  systems. 
It  is  a  fact  which  has  as  yet  received  too  little  attention  that 
such  fundamental  values  as  the  maximum  solubility,  the 
freezing  point  and  the  melting  point  of  a  substance  vary  in 
marked  fashion  with  the  size  of  the  particles  of  the  material 
concerned.  Thus,  as  far  as  known,  the  solubility  of  solids 
(in  molecularly  dispersed  form)  always  increases  with  increase 
in  the  degree  of  dispersion.  The  old  experiments  of  G.  STAS 
already  suffice  to  show  how  true  this  is.  STAS  found  the 
solubility  of  silver  chlorid  in  the  precipitated  form  (in  other 
words,  as  particles  of  approximately  colloid  size)  to  be  a 
hundred  times  greater  than  the  solubility  of  this  same  sub- 
stance when  granular  or  coarsely  dispersed.1  This  behavior 
is  not  peculiar  to  silver  chlorid,  but  holds  for  all  solid  sub- 
stances. In  analogous  manner,  the  freezing  point  of  water 
is  lowered,  not  only  through  the  presence  in  it  of  molecularly 
dispersed  phases  but  simply  by  being  allowed  to  freeze  in  a 
dispersed  form,  as  when  it  is  confined  in  capillary  spaces. 
The  water  which  has  been  allowed  to  soak  into  a  clay  sphere 
does  not  freeze  until  —0.7°  C.,  and  wet  filter  paper  does 
not  stiffen  until  —  0.1°  C.  is  reached.  These  are  not  simply 
phenomena  of  mere  under-cooling,  let  it  be  noted.  Looked 
at  from  the  ordinary  point  of  view,  these  facts  simply  indi- 
cate that  determinations  of  the  freezing  point,  were  they 

1  For  details  and  for  further  examples  see  WOLFGANG  OSTWALD,  Hand- 
book of  Colloid  Chemistry,  Trans,  by  FISCHER,  OESPER  and  BERMAN,  Phil- 
adelphia, 1915. 


CLASSIFICATION  OF  THE  COLLOIDS  73 

carried  out  in  capillary  tubes,  would  always  show  too  great 
values.  But  regarded  from  the  point  of  view  of  the  disper- 
soid  chemist,  they  cannot  help  but  suggest  that  the  lowering 
of  the  freezing  point  in  molecularly  dispersed  systems  is  also 
nothing  but  a  " capillary"  phenomenon;  to  him,  the  pres- 
ence of  molecules  and  ions  divides  the  solvent  into  many  tiny 
capillary  divisions  through  which  the  slight  depression  of 
freezing  point  noted,  for  instance,  in  the  coarsely  dispersed 
clay  sphere,  is  compounded  to  attain  the  great  values 
characteristic  of  molecularly  dispersed  solutions.1 

The  melting  point  of  a  substance  also  changes  with  the 
degree  of  its  dispersion.  It  is  lower  as  the  degree  of  disper- 

1  One  might  assume  that  the  depression  of  the  freezing  point  of  a  dis- 
persoid  is  proportional  to  its  internal  specific  surface,  in  other  words  to  the 
quotient  of  surface  of  the  dispersed  phase  (per  unit  weight)  and  the  volume 
of  the  dispersion  medium.  The  value  A  mol.  =  1.84°,  the  molar  depres- 
sion of  the  freezing  point  of  molecularly  dispersed  aqueous  systems,  would 
then  represent  the  capillary  depression  of  maximally  dispersed  systems 
(molecules  and  ions)  containing  one  mol  of  the  dispersed  phase  in  the  liter 
of  water.  In  such  maximally  dispersed  systems  matters  may  then  be  sim- 
plified by  substituting  for  the  specific  surface  the  concentration  of  the  dis- 
persed phase,  that  is  to  say,  the  number  of  particles  in  the  unit  volume, 
since  in  such  maximally  dispersed  systems  all  the  particles  have  practically 
the  same  size.  Such  conditions  obtain,  however,  only  in  extreme  cases  as 
in  "infinitely"  dilute  solutions  and  not  in  concentrated  solutions  nor  in 
such  as  are  admixed  with  polymeric,  associated  or  colloid  particles.  All 
the  latter  yield  too  small  depressions  of  the  freezing  point  as  compared  with 
the  molecular  solutions.  In  order  to  get  the  normal,  maximal  A  value  in 
such  instances,  a  larger  number  of  the  large  particles  must  be  present  in  the 
unit  volume. 

If,  for  simplicity's  sake,  we  assume  a  simple  proportion  to  exist  between 
specific  surface  and  freezing  point  depression,  the  specific  surface  (torn)  of 
molecular  particles  of  an  average  diameter  of  0.1  /*/*  to  the  specific  surface 
(fi/c)  of  colloid  particles  having  a  diameter  of  100  /*/*,  if  both  were  spheres, 
would  be  as  about  5.108  :  5.105  or  as  1000  :  1.  See  WOLFGANG  OSTWALD, 
Handbook  of  Colloid  Chemistry,  translated  by  FISCHER,  OESPER  and  BERMAN, 
91,  Philadelphia,  1915.  In  order  to  obtain  the  same  specific  surface,  1000 
times  as  many  colloid  particles  would  therefore  be  needed  in  the  same  unit 
volume  as  when  molecules  are  used. 

In  order  to  obtain  the  normal  or  maximal  depression  of  the  freezing  point 
in  water,  a  gram-molecule  (197.2  grams)  of  gold,  or,  roughly,  about  200  grams, 
would  therefore  have  to  be  molecularly  dissolved  in  1000  cc.  of  water.  A 
molar  solution  of  molecularly  dispersed  gold  would  therefore  contain  about 
20  percent  gold;  a  molar  colloid  solution  of  gold  possessed  of  the  same 


74  COLLOID  CHEMISTRY 

sion  is  increased.  Few  of  the  organic  chemists,  who  make 
such  melting  point  determinations  almost  daily,  know  how 
great  such  lowering  may  be.  P.  PAWLOW,  for  example, 
found  finely  powdered  salol,  antipyrin,  etc.  (but  in  which 
the  individual  particles  were  still  microscopically  visible) 
to  melt  at  a  temperature  7°  below  that  at  which  larger 
particles  did. 

200  X  1000 
specific  surface  would  have  to  contain  -  -^-^  -  ,  in  other  words,  200  parts 


of  gold  to  one  part  of  water.  This  means  that  not  until  we  dealt  with  a 
gold  mud  containing  99.5  percent  colloid  gold  and  about  0.5  percent  water 
would  we  obtain  a  depression  of  the  freezing  point  of  1.84°.  A  gold  mud 
made  up  of  particles  100  /x/x  in  diameter  and  containing  about  5  percent 
of  water  would  show  a  A  value  of  about  0.1°.  The  same  value  would  be 
shown  by  a  mud  containing  particles  10  /x/x  in  diameter  and  about  33  per- 
cent of  water.  On  the  other  hand,  a  2  percent  gold  sol  (which  belongs  to  the 
most  concentrated  of  the  gold  sols  that  can  be  produced  under  normal  cir- 
cumstances) would,  if  its  particles  were  100  MM  in  diameter,  be  expected  to 
show  a  depression  of  only  0.00018°;  if  the  particles  were  10  /xju,  a  depression 
of  0.00184.  These  are  values  which  correspond  with  those  obtained  ex- 
perimentally. 

If  we  assume  further  that  the  capillary  depression  of  the  freezing  point 
is  independent  of  the  specific  chemical  composition  of  the  dispersed  phase 
as  taught  in  the  classical  solution  theory,  a  dispersoid  like  wet  filter  paper 
which  shows  a  A  value  of  —  0.1°  and  is  possessed  according  to  H.  BECHHOLD 
of  pores  about  1  M  M  1000  MM  in  diameter,  would  have  a  concentration  of 
99  per  cent  solid  material  and  a  water  content  of  about  1  percent.  Sim- 
ilarly, a  dispersoid  of  the  type  of  the  clay  sphere  of  VON  BACHMETJEW,  to 
which  a  pore  value  not  exceeding  200  /x/x  may  be  attributed,  and  which  shows 
a  A  of  —  0.7°  would  have  a  concentration  of  99.3  percent  or  a  water  content 
of  about  0.7  percent.  As  a  matter  of  fact,  much  larger  amounts  of  water 
have  probably  been  absorbed  in  both  these  instances,  even  though  the  de- 
pression of  the  freezing  point  would  theoretically  be  expected  to  be  greatest 
when  the  water  content  is  lowest.  The  facts  argue  for  the  conclusion  that 
at  least  a  part  of  the  pores  and  spaces  in  filter  paper  and  clay  spheres  are 
decidedly  smaller  than  the  assigned  values.  This  conclusion  is  in  harmony 
with  the  observed  facts.  The  presence,  moreover,  of  molecularly  dissolved 
impurities  also  tends,  naturally,  to  give  the  observed  depressions  of  the  freez- 
ing point  fictitiously  high  values. 

These  remarks  are  merely  intended  to  indicate  the  possibilities  of  a  cap- 
illary theory  of  these  and  allied  phenomena.  For  the  detailed  development 
of  such  we  still  lack  quantitative  experiments  in  coarsely  dispersed  systems 
like  capillaries.  That  measurements  upon  such  coarsely  dispersed  systems 
may  be  used  after  the  fashion  of  the  well  known  experiments  of  J.  PERRIN 
for  the  calculation  of  the  constants  of  molecularly  dispersed  systems  can 
also  merely  be  pointed  out  here. 


CLASSIFICATION  OF  THE  COLLOIDS  75 

Imagine  such  changes  as  observed  in  these  relatively 
coarsely  dispersed  systems  to  be  continued  over  into  the 
realm  of  the  colloids.  It  seems  as  if  every  property  may  in 
this  realm  assume  a  new  value,  wherefore  we  need  not  be 
surprised  to  find  colloids  exhibiting  physico-chemical  re- 
actions which  the  coarsely  dispersed  material  shows  not  at 
all. 

According  to  what  is  known  as  WENZEL'S  law,  the  reaction 
velocity  of  solids  with  liquids  is  proportional  to  the  area  of 
contact.  It  need  not  surprise  us  to  find,  in  consequence, 
when  the  enormous  surface  presented  by  a  colloid  becomes 
available  for  reaction  purposes  — -  we  shall  return  to  this 
point  in  a  later  lecture  —  that  colloid  sulphur,  for  instance, 
acts  as  an  energetic  reducing  agent  for  silver  salts,  a  property 
which  coarsely  dispersed  sulphur  does  not  show  at  all;1  and 
that  colloid  platinum  still  decomposes  hydrogen  peroxid 
when  but  one  gram  atom  of  the  metal  is  present  in  seventy 
million  liters  of  the  dispersion  medium  (G.  BREDIG).  We 
shall  have  occasion  later  to  return  to  these  catalytic  effects. 
We  need  not,  however,  be  surprised  to  find  that  still  more 
highly  dispersed  metallic  colloids  prove  less  effective  in 
decomposing  hydrogen  peroxid,2  for  we  know  that  molecu- 
larly  or  ionically  dispersed  platinum  (such  as  platinic 
chlorid)  has  but  slight  or  no  catalytic  action.  We  observe 
again  a  curve  showing  a  maximum  and  one  of  the  same  type, 
therefore,  as  previously  noted  in  discussing  the  relation 
between  color  intensity  and  degree  of  dispersion.  Coarsely 
dispersed  and  molecularly  dispersed  metals  have  little  or  no 
catalytic  action,  while  those  colloidally  divided  work  most 
energetically.  At  a  certain  point  in  this  middle  region 
appears  the  maximum. 

§17. 

The  subject  of  the  relation  between  physico-chemical 
properties  and  degree  of  dispersion  is  very  great  and  I  could 

1  M.  RAFFO  and  A.  PIERONI,  Koll.-Zeitschr.,  7,  158  (1910). 

2  See  ST.  RUSZNYAK,  Zeitschr.  f.  physik.  Chem.,  86,  681  (1913). 


76  COLLOID  CHEMISTRY 

continue  much  longer  with  it,  but  perhaps  I  have  tired  you 
already.  If  so,  I  beg  you  to  remember  that  the  whole 
present  day  concept  of  the  colloids  stands  and  falls  with  the 
recognition  of  these  relations.  We  are  justified  in  regarding 
the  colloids  as  special  examples  of  the  dispersed  systems  only 
if  we  are  able  to  show  that  the  properties  of  colloid  systems 
blend  gradually  into  the  properties  of  the  coarsely  dispersed 
systems  on  the  one  hand,  and  into  those  of  the  molecularly 
dispersed  on  the  other.  This  new  concept  is  but  a  flimsy 
hypothesis  as  long  as  it  cannot  be  experimentally  proved. 

I  hope  that  I  have  succeeded  in  bringing  you  such  experi- 
mental proofs,  even  though,  as  I  have  emphasized,  many 
parts  of  the  subject  have  not  yet  been  worked  out  in  detail. 
The  primary  characteristic  of  a  colloid  is  its  special  degree  of 
dispersion.  If  this  is  true,  then  colloid  chemistry  becomes 
primarily  not  the  science  of  the  properties  of  a  special  group 
of  substances,  but  that  of  the  properties  of  a  physico-chemical 
state  into  which  any  substance  may  be  brought.  Like  the 
science  of  crystallography,  colloid  chemistry  deals  with  a 
special  physico-chemical  state  of  matter.  There  exists,  of 
course,  a  special  colloid  chemistry  also  which  details  the 
specific  variations  which  any  chemical  substance  may  show 
when  it  happens  to  appear  in  the  colloid  state,  just  as 
remarks  attached  to  a  discussion  of  any  chemical  substance 
which  are  crystallographic  in  character  may  be  found  in  our 
text-books.  Formerly  it  was  believed  that  colloid  chemistry 
had  spent  itself  when  it  had  thus  described  in  footnote 
fashion  the  colloid  properties  of  chemical  compounds. 
Today  we  insist  upon  the  existence  of  colloid  chemistry  as 
an  independent  division  of  the  physico-chemical  sciences.  It 
is  the  science  of  the  colloidally  dispersed  state.  The  estab- 
lishment of  this  fact  was,  in  brief,  the  main  purpose  of 
today's  lecture. 


III. 

THE  CHANGES  IN  STATE  OF  COLLOIDS. 


THIRD  LECTURE. 
THE  CHANGES  IN  STATE  OF  COLLOIDS. 

OUR  previous  lectures  have  dealt  for  the  most  part  with 
the  general  physical  chemistry  of  the  colloid  state,  or  of  the 
dispersed  state  in  general.  I  have  tried  to  show  you  where 
the  colloid  systems  fit  into  the  great  general  scheme  of  the 
dispersed  systems.  Today  we  shall  discuss  the  colloidally 
dispersed  systems  more  specifically.  This  does  not  mean, 
of  course,  that  these  possess  properties  for  which  there  are 
no  analogies  in  the  neighboring  realms  of  dispersion,  for,  as 
already  emphasized,  we  pass  gradually  from  the  coarse 
dispersions  into  the  colloids  and  through  these  into  the 
molecularly  dispersed  systems  —  but  a  whole  series  of 
phenomena  shows  itself  either  most  markedly  or  least 
intensely  in  the  colloid  realm  and  may  in  this  sense  be 
regarded  as  specific  for  the  colloid  state.*  These  phenomena 
will  absorb  our  chief  interest  today,  and  it  will  be  our  special 
problem  to  discover  what  special  changes  these  typical 
colloids  suffer  when  exposed  to  different  external  conditions. 
We  shall,  therefore,  start  with  colloid  systems,  expose  them 
to  different  experimental  conditions  and  see  what  happens. 
What  we  observe  may  justly  be  termed  the  special  physical 
chemistry  of  the  colloid  state. 


The  considerations  of  our  previous  lectures  again  permit 
us  to  predict  what  must  be  the  nature  of  a  large  number  of 
these  changes  in  the  colloid  state.  Ignoring  certain  radical 
changes  which  have  to  do  with  gross  destruction  of  the 
colloid  itself  by  chemical  means,  what  are  the  changes  which 
a  colloid  may  suffer?  It  may,  first  of  all,  undergo  changes 
in  its  degree  of  dispersion.  These  changes  may  be  limited 

79 


80  COLLOID  CHEMISTRY 

to  the  region  of  the  colloid  realm  itself,  or  they  may  extend 
beyond  this  into  the  realms  of  the  molecularly  dispersed  or 
the  coarsely  dispersed.  These  changes  which  occur  within 
the  limits  of  the  colloid  realm  itself  we  call  the  internal 
changes  in  state,  thus  distinguishing  them  from  the  radical 
changes  in  state  which  take  us  beyond  these  limits.  Of 
particular  importance  are  the  changes  in  degree  of  disper- 
sion occurring  at  the  limit  between  coarsely  dispersed  and 
colloid  systems.  Decreases  in  degree  of  dispersion  resulting 
in  coarsely  dispersed  systems  are  designated  coagulations. 
Liquid  colloids  are  also  known  as  sols;  their  coagulation 
products,  as  gels.  Increases  in  degree  of  dispersion  are 
designated  peptizations,  because  they  simulate  the  solution 
phenomena  observed  when  solid  proteins  are  acted  upon  by 
ferments.  Obviously  these  two  great  classes  of  changes  in 
state  can  be  foretold. 

§2. 

Another  great  group  of  changes  in  state  is  connected  with 
changes  in  the  type  of  the  dispersed  phase  in  colloids.  You 
will  recall  that  there  are  colloids  of  the  composition  solid  + 
liquid  (suspensoids)  and  liquid  +  liquid  (emulsoids).  We 
shall  consider  these  in  detail  today.  It  is  an  interesting  fact 
that  one  and  the  same  colloidally  dispersed  material  may 
vary  in  one  and  the  same  dispersion  medium  between  solid 
and  liquid;  as  we  shall  see  later,  the  colloid  particles  may 
pass  gradually  from  the  solid  to  the  liquid  state.  This  is 
particularly  true  of  the  hydrated  or  solvated  colloids,  in 
which  the  dispersed  particles  carry  about  with  them  more 
or  less  of  the  dispersion  medium  in  combined  form.  The 
process  is  analogous  to  the  behavior  of  a  piece  of  gum  arabic 
which  may  show  all  transitions  from  a  brittle  solid  to  a  liquid, 
depending  upon  the  amount  of  water  it  has  taken  up.  After 
I  have  showed  you  the  experiments  accompanying  today's 
lecture,  you  will  be  astonished  at  the  great  role  played  by 
these  changes  in  the  type  of  the  dispersed  particles.  The 
phenomena  of  gelation  and  of  swelling  belong  under  this 
heading. 


THE  CHANGES  IN  STATE  OF  COLLOIDS 


81 


§3. 

But  we  can  foresee  the  existence  of  even  a  third  class  of 
changes.  Were  you  ordered  to  sketch  a  colloid  you  would 
no  doubt  make  it  look  something  like  Fig.  28,  A.  You 
would  naturally  assume  that  the  colloid  is  distributed  uni- 
formly through  the  mass.  But  closer  study  shows  that  this 
is  only  approximately  true.  Wherever  the  colloid  comes  in 


O  O  O  O  O 

O  O  O  O  O 

O  O  O  O  O 

O  O  O  O  O 

O  O  O  O  O 

O  O  O  O  O 


B 

o  ooo 

oo  oo 

O 

0 

0      0  • 

O      0 

0 

0 

0      0 

0      0 

o 

0 

°000 

000° 

FIG.  28.  —  Diagram  illustrating  the  concept  of  adsorption. 

contact  with  a  surface  (as  the  vessel  wall  or  the  air)  it  fails 
to  maintain  a  uniform  spacial  distribution.  At  the  surfaces 
of  contact  the  concentration  becomes  different  from  that 
obtaining  in  the  inner  parts  of  the  colloid  mass.  The 
surfaces  contain  either  less  or  more  of  the  dispersed  phase 
than  the  rest  of  the  system,  as  indicated  in  Fig.  28,  B. 
Usually  the  colloid  tends  to  concentrate  in  the  surfaces. 
These  changes  in  concentration  are  commonly  designated  as 
adsorption.  It  was  the  American  giant,  WILLARD  GIBBS, 
who  first  pointed  out  that  at  the  surfaces  of  dispersed  sys- 
tems a  different  concentration  was  to  be  expected  than 
prevails  in  the  body  of  the  dispersoid.  GIBBS  did  not,  of 
course,  either  know  or  use  this  modern  concept  of  the 
dispersed  systems,  but  his  deductions  were  of  such  general 
nature  that  they  apply  also  to  the  special  field  which  we  are 
discussing.  Adsorption  phenomena  are  of  great  variety  and 
play  a  great  role  in  many  different  ways  in  the  changes  in 
state  of  colloids. 

It  must  be  remembered  that  changes  in  degree  of  disper- 
sion, hi  type  of  dispersed  phase  and  in  its  spacial  distribution, 


82  COLLOID  CHEMISTRY 

constitute  by  no  means  all  the  possible  changes  in  state. 
Neither  do  these  three  groups  of  phenomena  appear  singly; 
they  frequently  appear  in  combination.  The  many  com- 
binations possible  and  the  innumerable  resultant  changes  in 
state  constitute  the  prime  reasons  for  the  great  instability 
of  the  colloids.  Even  GRAHAM  said  that  rest  never  ruled 
hi  the  colloid  state.  These  facts  render  apparent  why  the 
scientific  study  of  the  changes  in  state  of  colloids  is  of  such 
great  interest  and  why  such  particularly  complicated 
phenomena  as  those  of  life  take  place  in  colloid  media  and 
only  in  such. 

§4. 

But  let  us  leave  these  theoretical  considerations  and  return 
to  the  experimental.  By  what  experimental  methods  may 
we  study  qualitatively  and  quantitatively  the  changes  in  the 
state  of  colloids?  A  first  fact  ever  to  be  borne  hi  mind  is 
that  changes  in  the  state  of  a  colloid  always  take  time; 
differently  expressed,  they  occur  at  definite  velocities.  They 
take  time  as  do  chemical  reactions,  but  differ  from  these  in 
that  their  end  states  are  not  so  clearly  defined.  The  end 
products  of  a  chemical  reaction  are  definite  chemical  com- 
pounds with  constant  properties;  the  changes  in  state  of  a 
colloid  may  halt  when  any  degree  of  dispersion  or  hydration 
has  been  attained.  The  kinetic  element,  therefore,  serves 
to  characterize  colloid  changes  in  state  even  more  markedly 
than  it  does  the  changes  observed  in  the  fields  of  "pure" 
chemistry.  It  is  for  this  reason  that  the  ideal  method  of 
studying  colloid  changes  always  proves  to  be  a  kinetic  one. 
But  it  also  follows  that  the  characteristics  of  gelation,  of 
swelling,  of  coagulation,  etc.,  will  never  be  marked  by  clearly 
defined  " points."  For  instance,  it  is  impossible  to  say  that 
a  protein  solution  coagulates  when  the  concentration  of 
twenty  percent  ammonium  sulphate  has  been  reached.  A 
"slight  turbidness"  is  likely  to  be  observable  at  ten  percent; 
a  "marked  turbidness,"  at  fifteen  percent;  a  "beginning 
precipitation,"  at.  seventeen  percent,  etc.  Nor  will  such  a 


THE  CHANGES  IN  STATE  OF  COLLOIDS  83 

protein  be  suddenly  coagulated  if  its  temperature  is  decreased 
a  degree.  The  expression  of  such  findings  in  the  terms  of 
kinetics  is  far  better.1 

In  the  case  of  the  simpler  changes  in  the  colloid  state,  as 
when  the  degree  of  dispersion  is  merely  altered,  we  may 
follow  the  changes  in  the  size  of  the  particles  by  employing 
the  ultramicroscope,  ultrafiltration,  etc.,  and  plotting  the 
results  as  a  velocity  curve;  or  we  may  utilize  the  principle 
discussed  in  a  previous  lecture  according  to  which  every 
physico-chemical  property  of  a  colloid  varies  with  its  degree 
of  dispersion.  In  this  way  I  demonstrated  to  you  the 
increase  in  the  size  of  the  particles  during  coagulation  in  a 
gold  sol  when  calling  your  attention  to  the  changes  in  color 
from  red  to  blue.  In  complicated  cases  we  still  have  recourse 
to  such  indirect  methods.  We  measure,  for  example, 
changes  in  conductivity,  in  turbidity,  in  viscosity,  etc.,  and 
then  from  their  value  parallel  them  with  changes  in  the  state 
of  the  colloid.  Of  course,  we  try  to  choose  properties  which 
are  easily  measured  and  of  such  prominence  as  to  change 
with  but  slight  alterations  in  state.  The  methods  must, 
moreover,  permit  a  study  of  the  changes  in  state  without 
causing  a  destruction  of  the  colloid  itself. 

We  observe  changes  in  the  internal  state  of  colloids  in 
great  variety  in  such  materials  as  gelatin,  albumin,  rubber 
or  cellulose.  We  have  called  these  colloids  sol  vat  ed  emul- 
soids  and  will  learn  more  of  them  shortly.  One  of  their  most 
conspicuous  physico-chemical  properties  and  one  intimately 
associated  with  their  internal  state  is  their  viscosity.  No 
better  example  of  its  importance  can  be  given  than  appears 
in  the  fact  that  the  FARADAY  SOCIETY  in  1913  devoted  a 

1  So  far  as  I  know,  I  was  the  first  to  emphasize  that  changes  in  state  should 
by  definition  receive  kinetic  treatment  instead  of  being  characterized  by  the 
customary  "points."  See  my  Grundriss  der  Kolloidchemie,  1.  Aufl.,  267, 
Dresden,  1909;  also,  Koll.-Zeitschr.,  12,  218  and  246  (1913);  Die  neuere 
Entwicklung  der  Kolloidchemie,  18,  Dresden,  1912.  Following  these  sug- 
gestions of  mine,  there  appeared  the  papers  of  H.  PAINE,  H.  FREUNDLICH, 
N.  ISHIZAKA,  etc.  See  Koll.-Zeitschr.,  11,  115  (1912);  Koll.-Zeitschr.,  12, 
230  (1913);  Kolloidchem.  Beihefte,  4,  24  (1912),  where  references  to  further 
papers  will  be  found. 


84 


COLLOID  CHEMISTRY 


whole  meeting  to  its  discussion.1  Over  a  dozen  colloid 
chemists  spent  two  sessions  on  this  special  theme.  Nor 
could  I  do  better  to  illustrate  the  variety  and  importance 
of  the  internal  changes  in  state  of  which  a  colloid  is  capable 
than  by  giving  you  an  outline  of  some  viscosity  studies. 

§5. 

Anyone  who  has  busied  himself  with  dialysis  or  diffusion 
experiments  soon  discovers  that  there  are  two  distinct 
classes  of  colloids  among  the  many  non-dialyzing,  non- 
diffusing  dispersoids  with  which  he  may  work.  Their 


Concentration »- 

FIG.  29.  —  Diagram  illustrating  the  relation  between  viscosity  and 
concentration  in  different  types  of  colloids. 

behavior  is  totally  different.  Especially  evident  is  the 
great  viscosity  of  the  one  as  compared  with  that  of  the 
other.  Colloids  of  gold  or  of  the  metallic  sulphids  hardly 
alter  the  viscosity  of  their  dispersion  media.  In  low 
concentration  these  colloids  are  as  mobile  as  their  "  sol- 
vents" by  themselves  and  even  in  high  concentrations 
their  viscosity  increases  are  only  arithmetric  in  type.  This 
is  shown  in  curve  A  of  Fig.  29.  The  behavior  is  char- 
acteristic of  the  solid  +  liquid  type  of  colloids,  the  sus- 
pensoids. 

Colloids  like  gelatin  which  we  said  above  are  hydrated 
emulsoids  behave  entirely  differently.     They  show  enor- 
i  See  Koll.-Zeitschr.,  12,  Heft  5  (1913). 


THE  CHANGES  IN  STATE  OF  COLLOIDS  85 

mous  viscosity  values  in  even  very  low  concentrations, 
and  with  every  increase  in  the  concentration  of  the  colloid, 
the  viscosity  value  mounts  enormously,  as  shown  in  curve 
B  of  Fig.  29.  As  you  know,  gelatin  in  water  shows  all 
possible  viscosity  values  from  that  of  pure  water  to  that 
of  a  solid  jelly  within  the  concentration  realm  of  a  two 
percent  solution.  We  know  other  emulsoids  which  will 
show  such  variations  from  unit  viscosity  to  infinity  (the 
viscosity  of  solids)  within  even  narrower  ranges  of  con- 
centration. Castor  oil  soap,  for  example,  is  an  almost  solid 
jelly  at  a  concentration  of  0.1  percent,  if  a  certain  amount 
of  alkali  is  present;  and  similarly  striking  figures  may  be 
cited  for  other  organic  colloids.1 

§6. 

Besides  these  great  absolute  and  relative  effects  of 
concentration  upon  viscosity,  other  factors  influence  it, 
as  temperature.  As  you  know,  the  viscosity  of  molecules 
decreases  steadily  with  increase  in  temperature.  The 
viscosity  of  pure  water,  for  instance,  decreases  some 
2  percent  for  every  degree  of  rise  in  temperature  between 
0°  and  25°  C.  But  in  gelatin  solutions  such  decrease  in 
viscosity  is  enormously  greater.  In  concentrated  solu- 
tions or  in  gels  the  decrease  is  so  great  within  certain  tem- 
perature limits  that  phenomena  are  observed  which  seem 
analogous  to  the  melting  of  homogeneous  solids.  Within 
a  temperature  range  of  but  a  few  degrees,  there  occurs  a 
fall  in  viscosity  from  the  values  characteristic  of  solids  to 
those  characteristic  of  fluids.  It  is  an  important  fact,  too, 
that  the  observed  change  is  a  progressive  one  and  not 
such  a  sudden  change  in  state  as  is  observed,  for  example, 
in  the  melting  of  a  crystal. 

High  viscosity  values  are  observed  even  in  non-solvated 
emulsoids.  Concentrated  sulphur  solutions,  for  example, 
may  show  a  salve-like  consistency.  There  exist  many 

1  Further  examples  are  detailed  in  the  paper  of  W.  DOHLE,  Koll.-Zeitschr., 
12,  73  (1913). 


86  COLLOID  CHEMISTRY 

theoretical  reasons  why  the  liquid  state  of  the  dispersed 
phase  should  alone  suffice  to  explain  on  a  physical  basis 
the  great  viscosity  of  the  emulsoids.1 

§7. 

Besides  concentration  and  temperature,  other  external 
conditions  alter  the  viscosity  of  solvated  emulsoids,  though 
less  effectively.  Simple  shaking  or  repeated  passage  through 
a  capillary,  for  instance,  often  suffice  to  decrease  viscosity. 
This  may  be  observed  in  milk.  The  viscosity  also  falls 
when  emulsoids  are  kept  for  a  time  at  higher  temperatures. 
In  fact,  time  alone  will  effect  viscosity  changes.  A  colloid 
needs  but  to  be  left  entirely  alone  for  a  time  in  order  that 
changes  in  its  viscosity  will  be  observed,  —  sometimes  an 
increase,  as  in  gelatin,  at  other  times,  a  decrease,  as  in 
starch. 

Of  course,  the  addition  of  extraneous  substances,  both 
electrolytes  and  non-electrolytes,  suffices  to  influence  the 
viscosity  of  colloids.  I  show  you  here  a  series  of  gelatin 
solutions  to  which  various  substances  have  been  added 
(demonstration).  The  first  tube  contains  a  pure  two  and 
one-half  percent  gelatin.  It  is,  as  you  see,  a  solid  jelly 
which  on  hard  shaking  separates  in  pieces  from  the  wall 
of  the  tube.  The  second  tube  contains  the  same  concen- 
tration of  gelatin,  but  to  it  have  been  added  several  per- 
cent of  dry  magnesium  sulphate.  The  jelly  is  decidedly 
stiffer  and  does  not  break  to  pieces  on  hard  shaking.  The 
sulphates,  citrates,  phosphates,  etc.,  all  increase  the  vis- 
cosity of  aqueous  colloids  of  the  type  of  gelatin,  and  of 
course,  to  a  much  greater  degree  than  when  added  to  pure 
water.  The  third  tube  again  contains  the  same  gelatin 
but  this  time  some  potassium  iodid  crystals  were  added 
to  it.  As  you  observe  when  I  tilt  the  tube,  the  gelatin 
has  remained  fluid.  lodids,  bromids,  cyanids  and  certain 
chlorids  decrease  the  viscosity  when  present  in  certain 

1  See  especially  E.  HATSCHEK,  KoU.-Zeitschr.,  7,  81,  301  (1910);  8,  34 
(1911);  11,  280,  284  (1912);  12,  283  (1913);  13,  88  (1913). 


THE  CHANGES  IN  STATE  OF  COLLOIDS  87 

concentrations.  The  remaining  tubes  show  the  effects  of 
some  added  non-electrolytes.  Chloral  hydrate  and  urea 
decrease  viscosity.  Alcohol  in  small  amounts  increases  it. 
This  problem  of  the  effect  of  added  substances  becomes 
much  complicated  by  the  fact  that  one  and  the  same  sub- 
stance may  either  increase  or  decrease  the  viscosity,  de- 
pending upon  the  concentration  in  which  it  is  added.  Thus, 
gelatin  solutions  to  which  acid  or  alkali  is  added  in  differ- 
ent concentrations  show  a  minimum  and  a  maximum  of 
viscosity,  while  when  chlorids  are  added  several  minima  and 
maxima  of  viscosity  may  be  noted. 

All  these  variations  in  viscosity  correspond  to  changes  in 
the  state  of  the  colloid  systems  themselves,  such  as  changes 
in  degree  of  dispersion,  in  type  of  dispersed  phase  or  in 
degree  of  solvation.  We  know,  for  example,  from  ultra- 
microscopic  and  other  means  of  investigation  that  in  the 
ageing  of  dilute  starch  there  occurs  not  only  a  decrease 
in  dispersion  but  also  a  dehydration  of  the  dispersed  phase. 
The  colloid  particles  not  only  give  off  a  part  of  their  water 
but  they  clump  to  form  larger  aggregates.  Here  a  decrease 
in  the  viscosity  parallels  a  decrease  in  dispersion  and  a 
passage  from  the  liquid  state  toward  the  solid.  Con- 
versely, the  addition  of  alkali  or  acid  in  certain  concen- 
trations may  increase  the  hydration  of  certain  colloids, 
as  those  of  protein.  Here  the  dispersed  phase  moves  from 
the  solid  toward  the  liquid  side,  a  change  which  again 
betrays  itself  by  an  increase  in  viscosity. 

With  other  colloids  we  are  not  yet  sure  what  special 
internal  changes  in  state  are  responsible  for  the  extreme 
variations  in  viscosity.  According  to  recent  studies  it 
seems  probable,  for  example,  that  gelatin  solutions  while 
cooling  develop  an  internal  structure;  in  other  words,  the 
coalescence  of  the  colloid  particles  yields  aggregates  that 
form  fibrils,  nets,  etc.  We  shall  return  to  this  question 
later.  The  effect  of  adding  certain  substances,  as  salts 
in  different  concentrations,  cannot  as  yet  be  connected  in 
any  satisfactory  way  with  the  accompanying  changes  in 


88  COLLOID  CHEMISTRY 

degree  of  dispersion  and  of  hydration.  To  get  light  here 
we  need  to  supplement  the  general  methods  of  physico- 
chemical  investigation  with  the  colloid-chemical  ones  of 
ultramicroscopy,  ultrafiltration,  etc. 

§8. 

Our  discussion  of  gelation  brings  us  into  the  field  of  the 
changes  in  state  which  occur  within  the  limits  defining  the 
realm  of  the  colloids  themselves.  Let  us  begin  by  asking 
what  happens  when  a  gelatin  solution  cools  and  sets.  What 
are  the  internal  changes  which  lead  to  the  enormous  in- 
creases in  viscosity  that  are  observed,  for  instance,  when 
the  originally  liquid  mass  gradually  changes  into  a  solid? 
Let  me  illustrate  the  theory  of  gelation  by  an  experiment. 

I  have  in  this  flask  two  liquids  which  at  ordinary  tempera- 
ture are  only  partially  soluble  in  each  other,  namely,  phenol 
containing  some  water,  and  water  containing  a  little  phenol. 
Even  at  a  distance  you  observe  that  they  scarcely  dissolve 
in  each  other,  for  as  soon  as  I  shake  the  flask  the  mass  of 
liquid  shows  the  white  color  of  an  emulsion  (demonstra- 
tion). The  solubility  of  phenol  and  water  in  each  other 
increases  greatly  with  increase  in  temperature.  To  prove 
this  I  heat  the  mixture  while  continuously  shaking  it 
(demonstration).  As  soon  as  I  have  warmed  the  mixture 
to  a  temperature  of  70°  C.  the  emulsion  clears;  in  other 
words,  the  amounts  of  phenol  and  water  with  which  I 
started  dissolve  completely  in  each  other.  I  could  actu- 
ally add  any  amount  more  of  either  of  the  two,  at  this 
temperature,  without  their  separating  out. 

Above  the  " critical"  temperature,  phenol  and  water  are 
miscible  in  all  proportions.  For  this  experiment  I  have, 
of  course,  not  chosen  indifferent  amounts  of  phenol  and  of 
water,  but  a  concentration  of  about  36  percent  of  phenol. 
This  represents  the  " critical"  concentration  of  phenol  in 
water,  the  significance  of  which  I  cannot  discuss  further  here. 

While  I  have  been  talking,  the  white  emulsion  of  phenol 
and  water  has  given  way  to  a  clear  solution.  We  have 


THE  CHANGES  IN  STATE  OF  COLLOIDS  89 

before  us  now  a  molecularly  dispersed  solution  of  the  phenol 
in  water.  But  the  phenomenon  to  which  I  wish  to  direct 
your  particular  attention  is  to  be  observed  when  we  begin 
to  cool  this  system,  either  by  shaking  the  flask  in  the  air 
or  by  placing  it  under  the  water  tap.  You  can  see  in  ad- 
vance that  as  I  reduce  the  temperature  a  separation  of 
the  system  must  again  occur,  for  the  solution  phenomena 
that  you  have  observed  are  entirely  reversible.  The  solu- 
tion has  now  cooled  somewhat,  but  if  you  observe  closely 
you  note  at  the  same  time  that  it  has  also  changed  in 
appearance.  What  was  formerly  a  completely  colorless 
liquid  shows  now  a  bluish  yellow  opalescence  entirely  iden- 
tical with  the  opalescence  of  an  egg  white  solution,  or  a 
highly  dispersed  mastic  colloid.  The  opalescence  increases 
as  the  mixture  cools.  The  similarity  between  the  opal- 
escence in  our  phenol-water  mixture  and  the  opalescence 
of  typical  colloids  compels  the  conclusion  that  we  have 
before  us  a  separation  of  the  phenol  in  the  water  in  colloid 
form.  In  fact,  consideration  of  the  problem  compels  the 
conclusion  that  such  a  colloid  state  must  be  passed  through 
in  the  course  of  the  separation  in  such  a  system;  and  it 
only  becomes  a  question  as  to  whether  we  are  able  experi- 
mentally to  maintain  or  " stabilize"  this  colloid  state, 
when  reached,  long  enough  to  be  able  to  examine  it  care- 
fully. We  begin  with  a  temperature  at  which  we  have 
to  do  with  a  molecular  division  of  the  two  substances  in 
each  other  and  end  with  a  lower  one  at  which  we  have  to 
do  with  a  coarsely  dispersed,  or  even  non-dispersed,  mix- 
ture of  phenol  and  water.  Evidently  somewhere  between 
these  two  extremes  we  pass  through  the  colloid  realm  and 
the  opalescence  of  the  mixture  before  you  renders  it  prob- 
able that  we  have  it  before  us  now.  Ultramicroscopic 
investigation  furnishes  direct  proof  that  this  opalescent 
critical  fluid  mixture  is  really  one  in  which  the  divided  phase 
exists  in  colloid  dimensions.1 

1  This  characterization  of  critical  fluid  mixtures  as  emulsoids  was  first 
made  by  me  in  the  Koll.-Zeitschr.,  1,  335  (1907);  see  also  my  Grundriss 
der  Kolloidchemie,  1.  Aufl.,  102,  Dresden,  1909. 


90 


COLLOID  CHEMISTRY 


Other,  and,  in  part,  startling  analogies  are  discoverable 
between  the  properties  of  critical  fluid  mixtures  and  the 
properties  of  emulsoids,  more  particularly  of  the  solvated 
emulsoids  of  the  type  of  gelatin.  Unfortunately,  I  cannot 
demonstrate  all  these  to  you.  Critical  liquid  mixtures 
frequently  show,  for  example,  the  properties  of  foaming, 
which  are  not  observable  in  molecular  solutions  of  these 
materials  as  obtained  at  higher  temperatures. 


Butyric  acid  -water 


Temperature 


FIG.  30.  —  Viscosity  of  a  critical  fluid  mixture. 

But  more  important  still  is  the  fact  that  with  the  ap- 
pearance of  the  opalescence  there  occurs  also  a  remarkable 
increase  in  the  viscosity  of  the  mixture.  As  you  know,  the 
viscosity  of  every  liquid  increases  on  cooling,  but  in  the 
case  of  critical  fluid  mixtures,  this  increase  in  viscosity  is 
abnormally  great  in  the  regions  of  the  critical  temperature, 
and,  after  the  opalescence  region  has  been  passed,  the 
viscosity  falls  again  even  though  the  system,  as  a  whole, 
is  then  at  a  lower  temperature  than  before. 

If  the  viscosity  of  the  phenol-water  mixture  before  you 
is  measured  while  being  cooled,  there  is  first  observed,  at 


THE  CHANGES  IN  STATE  OF  COLLOIDS  91 

the  temperature  at  which  opalescence  appears,  a  sudden 
increase  in  the  viscosity,  as  shown  in  Fig.  30.  The  maxi- 
mum viscosity  at  this  temperature  is  much  above  the 
viscosity  of  either  of  the  two  pure  components.  If  the 
mixture  is  further  cooled,  the  opalescence  disappears, 
giving  way  to  a  white  turbidness  characteristic  of  the 
relatively  coarse  dispersoids.  This  is  the  state  that  the 
phenol-water  mixture  has  now  assumed  (demonstration). 
If  we  measure  the  viscosity  of  this  white  emulsion,  we 
find  it  considerably  less  than  that  of  the  opalescent  colloid 
mixture.  Paralleling  this  appearance  and  loss  of  opal- 
escence we  find  the  viscosity  attaining  a  maximum  in  the 
realm  of  colloid  separation,  and  then  falling  again  as  shown 
in  Fig.  30. 

To  give  you  some  conception  of  the  quantitative  side  of 
these  changes  in  viscosity,  let  me  read  you  a  few  figures 
that  have  been  observed  in  the  carefully  studied  critical 
fluid  mixtures  of  isobutyric  acid-water.  While  the  viscosity 
of  pure  water  at  20.12°  C.  has  a  value  of  1.1245,  and  that 
of  pure  acid  at  20.08°  one  of  1.983,  the  viscosity  of  a  critical 
mixture  of  59.93  percent  acid  has  a  value  of  3.677  at  20.99°. 
The  changes  in  viscosity  become  still  more  evident  when 
the  logarithms  of  viscosity  are  compared  with  the  tem- 
peratures, as  in  the  accompanying  Fig.  30. 

This  analogy  between  the  behavior  of  critical  fluid  mix- 
tures and  that  of  solvated  emulsoids  throws  light  upon 
some  of  the  changes  that  take  place  in  the  process  of  gela- 
tion. The  sudden  increase  in  viscosity  of  a  critical  fluid 
mixture  when  cooled  is  analogous  to  the  stiffening  of  a 
solvated  colloid  and  the  process  of  gelation.  There  occurs 
in  gelation,  as  in  critical  fluid  mixtures,  a  separation  of 
the  colloid  —  a  conclusion  that  may  be  verified  by  ultra- 
microscopic  and  other  methods.1 

1  See  WOLFGANG  OSTWALD,  Grundriss  der  Kolloidchemie,  1.  Aufl.,  347, 
Dresden,  1909.  The  ultramicroscopic  phenomena  predicted  here  were  sub- 
sequently discovered  and  developed  by  R.  ZSIGMONDY,  W.  MENZ,  W.  BACH- 
MANN  and  others.  See  their  papers  in  the  Kolloid-Zeitschrif t . 


92  COLLOID  CHEMISTRY 


In  the  gelation  of  colloids,  such  as  gelatin,  agar,  albumin, 
etc.,  a  separation  into  two  well-marked  phases  takes  place. 
As  in  phenol-water  mixtures,  there  are  formed,  first,  a  con- 
centrated phase  rich  in  colloid  and  containing  little  water, 
and  second,  a  dilute  phase  rich  in  water  and  relatively  poor 
in  colloid.  This  is  true  of  those  colloids  at  least  which  in 
the  presence  of  sufficient  water  always  have  the  tendency 
to  pass  over  into  the  liquid  state  and  which  have  never 
been  observed  in  solid,  for  example,  crystalline  form.  It 
is  possible,  however,  for  phases  which  originally  separate 
out  as  fluid  droplets  to  yield  later  tiny  solid  crystals.  This 
occurs  in  the  course  of  time  in  the  gelation  of  silicic  acid 
and  certain  soap  solutions  for  instance.  Both  silicic  acid 
and  certain  soaps  originally  give  rise  to  gels  which  look 
much  like  the  ordinary  emulsoid  gels,  yet  they  are  dis- 
tinguishable from  these  by  not  possessing  much  elasticity. 
But  that  which  is  common  to  all  these  gelations  is  the 
phenomenon  of  separation;  in  other  words  the  decrease  in 
the  degree  of  dispersion  of  the  whole  system  on  the  one 
hand,  and  the  distribution  of  the  dispersion  medium  in 
unequal  concentration  in  the  two  phases  on  the  other. 

It  might  now  be  urged  against  this  comparison  that  in 
a  critical  And  mixture  the  separation  of  the  two  phases, 
when  the  dispersed  state  is  reached,  is  not  maintained,  but 
that  a  coarsely  dispersed  or  even  non-dispersed  mixture 
of  the  two  liquids  ultimately  comes  to  pass.  But  even 
this  has  found  its  analogue  in  jellying  colloids.  GRAHAM 
first  observed  it  and  his  findings  have  been  commented 
upon  many  times  since,  though  little  attention  has  been 
paid  to  these  comments.1 

1  I  have  during  the  past  years  made  many  experiments  upon  syneresis, 
without,  however,  having  found  time  to  bring  them  to  a  conclusion  and  to 
publish  them.  Not  only  the  term  itself  but  all  reference  to  syneresis  seems 
to  have  disappeared  from  the  literature.  None  of  the  ordinary  text-books 
of  colloid  chemistry,  for  example,  touch  upon  it.  I  hold  it  and  its  study 
as  extraordinarily  important,  as  may  be  inferred  not  only  from  what  is  said 
above,  but  from  some  paragraphs  which  are  to  follow.  The  phenomena 


THE  CHANGES  IN  STATE  OF  COLLOIDS  93 

When  any  gel  is  left  to  itself  for  a  number  of  hours  or 
days,  protected  not  only  against  infection  with  micro- 
organisms, but  also  against  evaporation,  a  separation  into 
two  phases  takes  place.  Every  bacteriologist  working  with 
solid  media  has  observed  this.  Agar  slants,  for  example, 
squeeze  off  fluid  droplets  in  the  course  of  time,  which 
coalesce  to  form  considerable  volumes  of  liquid.  The 
liquid  is  usually  called  condensation  water,  a  designation 
liable  to  confuse  one,  for  the  liquid  is  not  the  product  of 
condensed  water  vapor,  but  is  actually  secreted  by  the 
colloid.  The  liquid  is,  moreover,  not  pure  water  but  a 
solution  of  all  the  constituents  of  the  gel  in  both  colloid  and 
molecular  degrees  of  dispersion.1  Only  these  constituents  are 
present  in  a  different,  that  is  to  say  lower,  concentration. 
The  " serum"  given  off  is  therefore  in  reality  a  second 
colloid  solution  secreted  by  the  concentrated  colloid,  the 
whole  being  quite  analogous  to  the  separation  phenomena 
noted  in  critical  fluid  mixtures.  GRAHAM  called  this  sep- 
aration syneresis.  It  is  strange  how  little  this  theoreti- 
cally and  practically  important  phenomenon  has  been 
studied. 

I  have  never  seen  a  gel  which  does  not  show  syneresis. 
Not  only  do  agar  and  gelatin  (in  which  the  amount  of  the 
fluid  given  off  increases  with  decreasing  concentration 
of  the  colloid)  show  this  behavior,  but  so  do  starch,  silicic 
acid  (the  concentrated  gels  of  which  give  off  more  fluid 
than  the  dilute),  rubber,  collodion,  cellulose,  the  gelatinous 
sodium  chlorid  which  I  have  already  shown  you,  etc. 

I  show  you  here  the  syneresis  of  some  gelatin  and  sili- 
cic acid  gels  (demonstration).  In  order  to  prove  that 

of  syneresis  not  only  cover,  in  my  mind,  a  group  of  changes  in  state  coordinate 
with  the  phenomena  of  swelling  and  of  gelation,  but  they  are  possessed  of 
possibilities  for  application  to  scientific  and  technical  problems  of  the  great- 
est importance.  See,  in  this  connection,  the  last  two  lectures  in  this  volume. 
1  According  to  unpublished  experiments,  I  demonstrated  the  presence  in 
the  syncretic  serum  of  the  colloid  responsible  for  the  gelation  in  the  fluids 
expressed  by  silicic  acid,  gelatin,  agar,  starch,  sodium  chlorid  and  polymer- 
ized cinnamic  ethyl  ester.  The  degree  of  dispersion  of  the  colloid  in  the 
expressed  fluid  is,  naturally,  always  much  greater  than  in  the  gel. 


94  COLLOID   CHEMISTRY 

the  fluids  which  have  been  expressed  are  not  simply  water 
or  salt  solutions  but  contain  colloid  material  as  well,  I 
shall  pour  off  a  part  of  the  "serum"  from  each  of  the  two 
flasks.  To  the  liquid  from  the  gelatin  flask  I  add  a  few 
drops  of  dilute  hydrochloric  acid  and  some  tannin.  The 
white  cloud  which  develops  betrays  the  presence  of  gelatin. 
To  this  second  test  tube  I  add  some  copper  sulphate;  the 
precipitate  of  copper  silicate  formed  shows  that  the  " serum" 
in  this  case  also  contains  colloid  material. 
1  This  phenomenon  of  syneresis,  therefore,  closes  the  ring 
of  analogies  between  the  behavior  of  critical  fluid  mixtures 
and  that  of  hydrated  colloids.  Let  me  add  that  micro- 
scopic study  of  colloids  to  which  dehydrating  materials  have 
been  added  supports  the  assumption  that  separation  occurs 
in  " droplet"  form  in  the  process  of  gelation,1  and  that  this 
phenomenon  is  analogous  to  that  observed  when  benzol 
separates  as  a  coarse  emulsion  from  water. 

§10. 

But  the  gel  state  may  be  reached  not  only  by  allowing  a 
liquid  mixture  to  set,  but  by  allowing  a  solid  to  absorb  a 
dispersing  medium.  As  you  know,  a  disc  of  solid  gelatin 
goes  over  into  a  jelly  when  it  is  thrown  into  water.  Differ- 
ently expressed,  the  gelatin  swells.  Such  swelling  may  be 
observed  in  other  colloids  than  gelatin  and  with  dispersion 
media  other  than  water.  Thus,  rubber  swells  in  benzol, 
collodion  in  alcohol-ether,  etc.  Let  me  illustrate  some  of 
these  swellings  and  in  connection  with  them  call  your 
attention  to  the  more  important  properties  of  the  observed 
colloid  changes  in  state. 

I  have  here  a  disc  of  ordinary  carpenter's  glue,  the  under 
half  of  which  has  lain  in  water  over  night  (demonstration). 
You  observe  the  considerable  increase  in  volume  of  the 
immersed  portion,  and,  at  the  same  time,  an  interesting 

1  See  especially  the  studies  of  O.  BUTSCHLI  and  W.  B.  HARDY  discussed 
by  WOLFGANG  OSTWALD,  Grundriss  der  Kolloidchemie,  1.  Aufl.,  350,  Dresden, 
1909. 


THE  CHANGES  IN  STATE  OF  COLLOIDS 


95 


optical  change.  The  swollen  half  is  white  and  turbid  while 
the  unswollen  portion  has  kept  its  brown  color  and  relative 
transparency.  The  immersed  portion  has  yielded  a  jelly 
with  corrugated  surface,  well- 
marked  elastic  properties,  etc. 
(demonstration). 

The  increase  in  volume 
when  materials  swell  can  be 
demonstrated  even  better  in 
the  following  fashion.  I  have 
here  some  very  thin  and  but 
slightly  vulcanized  rubber  foil 
such  as  dentists  and  surgeons 
use.  I  have  cut  from  it  a 
pantaloon-shaped  strip  as  in- 
dicated in  A  of  Fig.  31.  One 
of  these  two  divisions  we  will 
now  permit  to  swell  in  order 
that  we  may  compare  its 
change  in  volume  with  that  of 
the  unswollen  portion.  To  do 
this  one-half  of  the  divided 
strip  is  left  hanging  outside  a 
test  tube  while  the  other  half  is 
tucked  inside.  I  next  carefully  fill  the  test  tube  with  cumol, 
or  benzol.  Since  the  swelling,  like  all  other  colloid  changes, 
takes  time,  I  set  the  tube  aside  for  a  few  minutes. 

The  production  of  a  gel  through  swelling  necessitates  the 
existence  of  certain  physico-chemical  relationships  between 
the  material  undergoing  swelling  and  the  liquid  producing  it. 
Of  the  reasons  for  this  we  know  but  little  as  yet.  Gelatin 
swells  in  water  but  not  in  benzol;  vulcanized  rubber  in 
benzol  but  not  in  water.  Sometimes  a  definite  temperature 
is  necessary  in  order  that  swelling  may  take  place.  Starch, 
for  example,  does  not  swell  at  room  temperatures  but  does 
at  higher  ones;  potato  starch  begins  to  swell  between  57°  and 
58°  C.  This  swelling  temperature  can  be  determined  very 


FIG.  31.  —  Swelling  of  soft  rubber. 


COLLOID   CHEMISTRY 


accurately 1  by  measuring  the  viscosity  of  a  starch  suspension 
while  it  is  being  heated.  As  soon  as  the  proper  temperature 
is  reached,  a  sudden  great  increase  in  the  viscosity  is  noted 
which  becomes  particularly  evident  if  the  logarithms  of  the 
viscosities  are  compared  as  in  Fig.  32. 


Hydration  of  starch 


Temperature- 


FIG.  32.  —  Changes  in  viscosity  during  the  hydration  of  starch. 

It  is  an  interesting  fact  that  certain  crystals,  like  those  of 
albumin,  show  well-marked  swelling.  Inorganic  solids  also 
swell,  as  do  even  metals  like  sodium  and  potassium  in  the 
presence  of  liquid  or  gaseous  ammonia.  At  low  tempera- 
tures all  these  substances  swell  considerably  without  losing 
their  general  forms.  When  the  ammonia  is  driven  off,  the 

1  See  WOLFGANG  OSTWALD,  Koll.-Zeitschr.,  12,  218  (1913).  Regarding 
optical  methods  of  determining  these  swelling  points,  see  M.  SAMEC,  Ivol- 
loidchem.  Beihefte,  3,  129  (1911). 


THE   CHANGES  IN  STATE  OF  COLLOIDS  97 

pure  solids  or  metals  are  again  obtained.  On  the  other 
hand,  when  the  ammonia  is  allowed  to  act  long  enough  all 
these  substances  liquefy,  yielding  at  first  a  doughy,  highly 
viscid  mass  which,  in  most  cases,  then  goes  over  into  a 
colloid  solution.1  The  same  thing  happens  when  acacia  or 
rubber  swells.  Under  the  influence  of  the  material  produc- 
ing the  swelling  and  especially  at  higher  temperatures,  the 
swelling  gradually  gives  way  to  colloid  solution.  In  some 
colloids,  as  in  acacia,  the  temperature  necessary  for  this  lies 
so  low  that  at  room  temperature  only  solution  phenomena 
may  be  observed.  But  at  0°  C.  the  particles  of  acacia  show 
the  normal  phenomena  of  swelling. 

While  I  have  been  speaking  our  experiment  on  the  swelling 
of  rubber  has  made  progress,  and  so  I  now  withdraw  the 
immersed  half  from  the  test  tube.  As  you  see  (Fig.  31,  B), 
the  amount  of  swelling  even  in  these  few  minutes  is  enor- 
mous. The  immersed  half  is  at  least  half  again  as  long 
and  as  broad  as  the  unimmersed.  Moreover,  the  rubber  in 
swelling  has  changed  in  some  of  its  properties.  As  I  shake 
the  strip  you  catch  a  sound  not  unlike  that  heard  when 
writing  paper  is  rattled.  Please  note  the  relatively  great 
velocity  at  which  these  changes  in  state  have  taken  place. 
It  is  a  matter  of  enormous  interest  in  certain  biological 
problems  to  which  we  shall  return  later. 

in. 

Swelling  can  occur  not  only  in  liquids  but  also  in  vapors. 
But  there  is  a  difference  in  the  rate  at  which  the  swelling  is 
brought  about,  and  in  the  maximum  amounts  of  moisture 
that  may  be  taken  up  by  the  swelling  substances  in  the  two 
media.  A  gelatin  disc,  for  example,  takes  up  less  fluid  in  an 
atmosphere  of  water  than  in  water  itself. 

I  should  like  now  to  call  your  attention  to  another  swelling 
phenomenon  which  illustrates  swelling  in  vapor  and  which 

1  Regarding  the  swelling  of  metals  and  their  salts  in  ammonia  and  its 
probable  colloid  nature,  see  WOLFGANG  OSTWALD,  Kolloidchem.  Beihefte, 
2,  437  (1911)  where  references  will  be  found  to  the  literature. 


98  COLLOID  CHEMISTRY 

also  takes  place  with  incredible  speed.  The  experiment 
illustrates  the  close  association  between  swelling  and  certain 
kinetic  processes.  I  have  here  some  thin  leaves  of  colored 
gelatin  which  will  recall  your  childhood  days.  I  take  one 
that  has  been  cut  and  decorated  to  represent  a  fish,  and  lay 
it  upon  a  piece  of  filter  paper.  When  I  breathe  upon  it  you 
observe  that  the  gelatin  leaf  immediately  bends  or  even 
curls  up  and  jumps  upward  (demonstration).  Perhaps  you 
think  that  the  movement  is  merely  the  mechanical  result  of 
my  having  blown  upon  the  leaf.  To  meet  this  criticism  I 
have  fastened  a  gelatin  leaf  into  a  ring  stand.  When  I 
breathe  upon  it,  the  leaf  bends  away  from  me  (demonstra- 
tion). It  remains  for  a  time  in  the  new  position  and  only 
slowly  returns  to  the  old.  This  proves  that  the  movement 
is  really  brought  about  through  an  increase  in  the  volume 
of  the  gelatin  on  the  side  of  the  leaflet  which  has  been 
breathed  upon,  in  other  words,  through  swelling.  But  the 
swelling  soon  disappears,  for  the  absorbed  water  evaporates 
rather  quickly  from  so  large  a  surface. 

This  experiment  demonstrates  vividly  the  rapidity  and 
the  enormous  value  of  swelling  phenomena.  It  is  for  this 
reason  that  they  serve  us  in  the  making  of  certain  scientific 
measurements.  The  principle  is  used  in  the  well  known 
hair  hygrometer  in  which  the  change  in  the  length  of  a 
stretched  human  hair  through  the  humidity  of  the  air 
constitutes  the  principle  employed  for  measuring  the  latter. 
Why  blond  hair  works  better  for  these  purposes  than  dark 
hair  has  not  yet  been  settled  scientifically. 

§12. 

Of  great  interest  is  the  effect  of  the  addition  of  various 
substances  upon  swelling.  Electrolytes  and  non-electrolytes 
are  known  which  not  only  favor  but  also  inhibit  swelling. 
Among  the  most  powerful  of  those  which  increase  the  swelling 
of  such  colloids  as  are  capable  of  absorbing  water  are  acids 
and  alkalies.  Gelatin  and  fibrin,  for  instance,  in  the  pres- 
ence of  acids  or  alkalies  will  absorb  several  times  as  much 


THE  CHANGES  IN  STATE  OF  COLLOIDS  99 

water  as  when  these  are  absent.  Cyanids,  iodids,  chlorids, 
etc.,  also  favor  colloid  swelling  in  certain  concentrations, 
while  sulphates,  citrates,  phosphates,  alcohol,  sugar,  etc., 
do  the  opposite.  On  the  other  hand,  when  salt  and  acid  are 
used  in  combination,  the  addition  of  salt  usually  brings 
about  a  decrease  in  swelling,  even  though  the  salt  is  of  a 
kind  which  alone  leads  only  to  increased  swelling. 

To  demonstrate  these  effects  I  have  here  a  series  of  dishes 
containing  water,  acid,  alkali,  potassium  iodid,  calcium 
chlorid  and  magnesium  sulphate.  Into  each  of  them  has 
been  dropped  a  gelatin  disc  of  standard  weight  (demonstra- 
tion). To  render  the  discs  readily  visible,  a  trace  of  colloid 
dye,  namely,  congo  red,  has  been  mixed  with  the  gelatin. 
Had  a  molecularly  dispersed  dye  been  used  it  would,  of 
course,  have  diffused  out  of  the  gelatin  while  this  was 
swelling.  The  gelatin  discs  have  been  in  these  solutions 
some  twenty-four  hours  and,  as  you  observe,  the  way  in 
which  the  different  substances  have  favored  the  swelling  is 
as  follows : l 

acid,  alkali,  potassium  iodid,  calcium  chlorid,  water,  magnesium  sulphate. 

The  disc  in  the  acid  solution  is  already  swollen  to  twice  the 
size  of  that  in  the  pure  water. 

I  should  like  briefly  to  emphasize  that,  in  the  process  of 
swelling,  considerable  heat  is  set  free  and  that  considerable 
quantities  of  energy  suffer  conversion.  Swelling  seeds,  for 
example,  can  lift  great  weights.  In  a  skull  filled  with  dried 
peas  and  immersed  in  water,  the  swelling  peas  burst  the 
bones  of  the  head  apart.  The  old  Egyptians  used  these 
swelling  phenomena  for  quarrying  purposes  when  they  drove 
dry  wood  wedges  into  the  rocks  and  then  made  the  wedges 
swell  by  pouring  water  upon  them. 

1  These  gelatin  discs  are  prepared  by  pouring  a  concentrated  gelatin 
solution  upon  a  glass  plate.  The  gelatin  is  allowed  to  set,  after  which  it  is 
cut  into  squares  which  are  then  dried  and  weighed.  For  demonstration  pur- 
poses the  discs  are  immersed  in  ^o  to  Mo  normal  hydrochloric  acid  or  sodium 
hydroxid.  Half  normal  potassium  iodid,  ya  normal  calcium  chlorid,  or  highly 
concentrated  (saturated)  magnesium  sulphate  solution  may  be  used  to 
demonstrate  the  salt  effects. 


100  COLLOID  CHEMISTRY 

§13. 

Were  you  now  to  ask  me  what  are  the  internal  changes  in 
state  which  permit  a  solid  body  and  a  liquid  or  vapor  to 
become  a  jelly,  I  could  not  give  you  a  short  and  simple 
answer.  I  can  only  emphasize  the  following.  There  is,  in 
the  first  place,  a  great  similarity  between  a  swelling  system 
and  a  syncretic  emulsoid  system.  In  the  swelling  of  the 
colloid,  as  I  need  to  emphasize  at  this  time,  a  small  portion 
of  the  swelling  substance  always  passes  over  into  a  colloid 
solution.  In  syneresis,  on  the  other  hand,  we  have  also  to 
do  with  a  concentrated,  practically  solid  phase  covered  by 
a  dilute  colloid  solution.  Swelling,  in  other  words,  repre- 
sents the  reverse  of  syneresis.  The  whole  is  analogous  to 
what  may  be  observed  when  a  gelatin  gel,  which  has  been 
kept  at  a  low  temperature  and  which  shows  the  phenomenon 
of  syneresis,  is  again  exposed  to  a  higher  temperature.  In 
both  instances  a  pronounced  diphasic,  non-dispersed  system 
gives  way  to  a  simple  dispersed  system,  namely,  the  gel.  In 
the  process  of  swelling,  two  macroscopically  differentiated 
systems  give  rise  to  a  single  dispersed  one. 

The  power  of  a  solid  body  to  yield  a  gel  through  swelling 
seems  to  be  connected  with  the  presence  of  a  definite  struc- 
ture. Through  the  extensive  and  masterful  studies  of  0. 
BUTSCHLI,  G.  QUINCKE  and  others,  it  has  been  proved  that 
microscopic  and  ultramicroscopic  discontinuities  —  in  the 
form  of  meshes,  cells,  foams,  honeycombs,  etc.  —  are  widely 
distributed  in  nature.  Not  only  do  those  solids  which  are 
capable  of  swelling  show  these  structures,  but  also  many 
of  the  inorganic  crystals  as  those  of  congealed  sulphur 
and  the  so-called  skeletons  of  potassium  permanganate  and 
ammonium  chlorid.  This  finding  is  in  harmony  with  the 
fact  cited  above  that  these  crystalline  substances  show  a 
behavior  closely  allied  to  the  phenomena  of  swelling.  That 
metals  may  show  such  structure  is  well  known  to  every 
metallurgist. 

If  we  believe  with  0.  BUTSCHLI,  G.  QUINCKE  and  others 
that  structure  is  necessary  before  swelling  can  occur  in  a 


THE  CHANGES  IN  STATE  OF  COLLOIDS  101 

solid,  then  the  process  of  swelling  consists,  in  the  first  place, 
of  an  increase  in  the  degree  of  dispersion  of  these  systems. 
This  increase  is  then  analogous  to  the  increase  in  degree  of 
dispersion  as  observed  in  the  reversion,  through  heat,  of  a 
syneresis,  or  of  the  effects  of  age  in  a  colloid.  The  coarser 
structure  of  the  solid  is,  as  it  were,  broken  up;  in  other 
words,  coarse  aggregates  are  divided  into  the  primary 
particles  of  which  they  are  composed.  As  N.  GAIDUKOW 
has  found,  the  ultramicroscopic  particles  of  a  gel  become 
smaller  in  the  process  of  swelling,  or  at  least  lose  their  highly 
refractive  character.1  But  in  the  process  of  swelling  there 
occurs  another  change  which  may,  under  certain  circum- 
stances, actually  run  counter  to  the  increase  in  dispersion. 
The  individual  particles  absorb  the  medium  in  which  they 
are  swelling;  they  become  solvated.  This  increases  the  size 
of  the  particles  and  so  fluid  droplets  may  be  formed.  The 
two  changes,  in  other  words,  the  combination  of  increase 
in  degree  of  dispersion  with  a  change  in  the  type  of  the 
dispersed  substance  from  the  side  of  the  solid  to  that  of  the 
fluid,  seem  most  characteristic  of  the  process  of  swelling. 
Accordingly,  the  process  of  swelling  represents  the  reverse  of 
syneresis.  These  statements  cover  what  is  best  established 
at  this  time  regarding  the  changes  of  a  disperso-chemical 
nature  that  occur  in  the  process  of  swelling. 

§14. 

Let  me  add  a  few  words  regarding  the  properties  of  gels. 
Gels  show  in  interesting  fashion  the  properties  of  both  solids 
and  liquids.  Thus,  even  when  holding  98  percent  or  more 
of  fluid,  they  may  still  show  maintenance  of  form  and 
elasticity.  They  may  be  cut  or  broken  into  pieces  that  hold 
their  shapes,  and  may  be  bent  and  regain  their  original  form 
in  the  same  way  that  solids  do  (demonstration).  On  the 
other  hand  gels  show  the  properties  of  liquids.  Thus,  they 
flow  when  subjected  to  long-continued  deformation,  as 

1  See  N.  GAIDUKOW,  Dunkelfeldbeleuchtung  und  Ultra-mikroskopie  in 
der  Biologie  und  in  der  Medizin,  Jena,  1910;  Koll.-Zeitschr.,  6,  260  (1910). 


102  COLLOID  CHEMISTRY 

when  a  solid  two  percent  gelatin  cake  comes  to  assume  the 
shape  of  the  conical  vessel  into  which  it  is  crowded.  Un- 
fortunately this  experiment  takes  too  long  to  permit  me  to 
demonstrate  it  to  you.  The  liquid  character  of  a  gel  is  best 
betrayed,  however,  by  the  fact  that  molecularly  dispersed 
materials  diffuse  into  and  through  it,  as  already  discussed 
in  our  experiments  on  diffusion.  How  are  we  to  understand 
this  remarkable  combination  of  solid  with  liquid  properties? 

Our  previous  remarks  led  to  the  conclusion  that  gels  are 
systems  which,  on  the  one  hand,  are  more  coarsely  dispersed 
than  the  liquid  sols  from  which  they  are  prepared,  while  they 
are,  on  the  other  hand,  more  highly  dispersed  than  the  solids 
from  which  they  may  be  prepared  through  swelling.  The 
gels  evidently  occupy  a  middle  position,  so  far  as  degree  of 
dispersion  is  concerned.  Furthermore,  the  degree  of  dis- 
persion in  a  gel  may  vary  steadily  from  values  approximat- 
ing those  of  the  coarse  dispersions  to  those  of  the  colloids. 
Through  not  considering  this  fact  much  useless  debate  has 
arisen.  It  is,  for  example,  of  no  purpose  to  ask  whether 
gels  have  only  a  microscopic  or  only  an  ultramicroscopic 
structure,  since  both  may  be  found  not  only  in  different 
gels,  but  at  one  and  the  same  time  in  any  gel.  As  0. 
BUTSCHLI  assumed  and  as  proved  in  the  ultramicroscopic 
investigations  of  R.  ZSIGMONDY,  many  gels  have  a  dual 
structure.  They  may  contain  ultramicroscopic  particles 
which,  in  their  turn,  may  coalesce  to  give  rise  to  structures 
of  microscopic  dimensions.  This  behavior  is  common  and 
entirely  in  harmony  with  the  theory  of  gelation  sketched 
above. 

A  gel  may,  moreover,  actually  have  a  structure  of  micro- 
scopic dimensions  without  this  being  recognizable  by 
microscopic  or  ultramicroscopic  means.  As  I  emphasized 
before,  a  difference  in  degree  of  refraction  is  necessary  to 
permit  optical  differentiation.  In  highly  solvated  colloids 
such  differences  may  not  appear.  Nevertheless,  a  gel 
remains  a  gel  even  though  it  appears  optically  homogeneous, 
as  in  the  case  of  gelatin.  Sometimes  through  treatment 


THE  CHANGES  IN  STATE  OF  COLLOIDS  103 

with  alcohol  or  other  dehydrating  agents,  we  can  bring  about 
refraction  differences  in  such  gels,  which  then  make  possible 
optical  differentiation  of  structure.  By  the  use  of  such 
agents  we  may  change  quantitatively  the  characteristic 
properties  of  a  gel,  as  its  maintenance  of  form,  its  elasticity, 
its  permeability  to  molecular  dispersoids,  etc.,  but  we  do 
not  change  its  qualitative  character.  The  fact  remains  that 
the  degree  of  dispersion  of  a  gel  is  less  than  that  of  the  liquid 
solution  and  higher  than  that  of  the  solid  from  which  it 
was  derived.  Between  these  limits  the  degree  of  dispersion 
may  assume  any  value.  When  gels  are  produced,  as  is 
most  commonly  the  case,  by  allowing  colloid  solutions  to 
cool,  we  may  say  that  the  characteristic  structure  of  these 
systems  depends  upon  the  size  of  the  " primary"  colloid 
particles  composing  it,  which  means  that  it  must  at  least  lie 
near  the  realm  of  colloid  dimension  or  actually  within  that  of 
microscopic  visability.  Whether  in  such  gels  the  possibility 
always  remains  of  recognizing  these  "primary"  colloid 
particles  (as  by  ultramicroscopic  means)  is  a  question  of 
secondary  importance,  which,  in  different  cases  and  with 
different  degrees  of  dispersion  and  of  solvation,  may  have 
different  answers. 

But  just  as  the  degree  of  dispersion  may  vary,  so  also  may 
the  type  of  the  elements  composing  the  gel.  In  gels  produced 
by  swelling,  I  do  not  know  of  an  instance  in  which  the  dis- 
persed elements  are  solid  or  crystalline  in  character.  They 
are,  apparently,  always  liquid.  There  are,  however,  cases 
in  which  an  ultimate  separation  in  solid  crystalline  form 
occurs.  Thus,  the  gelatinous  precipitates  of  metal  hydroxids 
may  after  a  time  lose  their  typical  gel  properties  and  become 
crystalline.  This  can  also  be  noticed  in  silicic  acid  gels 
which,  in  the  course  of  time,  become  brittle  and  break  up 
into  inelastic  masses.  A  marked  elasticity  is  usually  more 
noticeable  in  emulsoid  than  in  suspensoid  gels,  though  in  this 
latter  case  the  crystalline  elements,  through  union  with 
their  solvents,  occasionally  make  possible  the  attainment 
of  considerable  elasticity. 


104 


COLLOID   CHEMISTRY 


§15. 

I  leave  this  somewhat  theoretical  discussion  in  order  to 
acquaint  you  with  some  further  properties  of  gels  (demon- 
stration). Since  gels  are  permeable  to  molecular  disper- 
soids,  chemical  reactions  may  be  permitted  to  occur  in  gels 
by  allowing  two  molecularly  dissolved  substances  to  diffuse 
toward  each  other  in  a  gel.  If  I  half  fill  a  test  tube  with  a 
gelatin  solution  containing  some  potassium  bichromate  and, 
after  the  gelatin  has  set,  pour  some  silver  nitrate  solution 
upon  it,  the  two  dissolved  substances  diffuse  into  each  other. 
The  silver  nitrate,  particularly,  will  diffuse  downwards  into 
the  gel,  as  I  have  previously  demonstrated  to  you.  But 
here  the  silver  salt  meets  the  bichromate  and,  exactly  as  in 
free  diffusion,  a  precipitate  of  silver  chromate  is  formed, 
the  amount  of  which  increases  as  the  diffusion  progresses. 

But  something  happens  in  this  re- 
action as  it  occurs  within  a  gel 
which  does  not  take  place  when 
diffusion  is  "free."  Were  no  gel 
present,  the  amount  of  the  precipi- 
tate would  increase  progressively 
with  the  course  of  the  reaction. 
But  in  the  presence  of  the  gel,  when 
the  conditions  for  the  experiment 
are  properly  chosen,  there  occurs  a 
periodic  precipitation.  In  the  test 
tubes  that  I  pass  around,  instead  of 
a  continuous  column  of  silver  chro- 
mate, you  observe  a  series  of  rings 
or  layers  of  this  substance  between 
which  the  gel  is  practically  colorless, 
indicating  the  absence  of  any  pre- 
cipitated silver  salt. 

Similar  periodic  precipitations 
may  be  obtained  by  allowing  other  reactions  to  occur 
in  gels.  In  Figs.  33  and  34  are  shown  some  beauti- 


FIG.  33.  —  Periodic  precipi- 
tations of  lead  chromate  ac- 
cording to  E.  HATSCHEK. 


THE  CHANGES  IN  STATE  OF  COLLOIDS 


105 


FIG.  34.  —  Periodic  precipitations  of  calcium  carbonate  according 
to  E.  HATSCHEK. 


FIG.  35.  —  Periodic  precipitations  of  silver  chromate  in  gelatin  accord- 
ing to  R.  E.  LlESEGANG. 


106 


COLLOID   CHEMISTRY 


fill  precipitates  of  lead  chromate  and  calcium  carbonate 
prepared  by  E.  HATSCHEK.1 

I  can  demonstrate  this  phenomenon  of  periodic  precipita- 
tion still  better  by  utilizing  a  projection  apparatus  and  some 
plate  preparations.  In  these  the  gel  containing  one  salt  has 
been  poured  upon  a  glass  plate.  After  setting,  the  second 
reacting  solution  has  been  dropped  upon  it  in  one  spot  or 


FIG.  36. — Silver  chromate  rings  in  gelatin. 

has  been  painted  upon  the  gel  in  the  form  of  a  ring  (Figs. 
35,  36  and  37).  You  observe  in  Figs.  35  and  36  how  the 
spot  made  by  the  original  drop  is  surrounded  by  a  vast 
number  of  dark  rings  marking  the  periodic  precipitations  of 
silver  chromate.  Fig.  37  shows  the  results  when  a  ring  is 
painted  upon  the  gelatin. 

These  periodic  precipitations  in  gels  are  known  as  LIESE- 

1  See  E.  HATSCHEK,  Koll.-Zeitschr.,  8,  193  (1910);  9,  97  (1910);  10,  77, 
124,  265  (1911);  14,  115  (1914);  also  the  many  papers  of  R.  E.  LIESEGANG, 
E.  KUSTER  and  others  in  the  Kolloid-Zeitschrift. 


THE  CHANGES  IN  STATE  OF  COLLOIDS 


107 


GANG  rings  in  honor  of  their  discoverer.  The  theory  of  their 
origin  is  still  a  matter  of  debate.  Even  the  pretty  and  long 
accepted  explanation  of  WILHELM  OSTWALD  seems  inade- 


FIG.  37.  —  Silver  chromate  rings  in  gelatin. 

quate  in  the  light  of  newer  studies.1  Should  any  of  you  like 
to  make  some  of  these  interesting  preparations  for  your- 
selves, let  me  emphasize  that  the  phenomenon  is  obtained 
in  good  form  only  when  definite  concentration  relationships 

1  Sec  especially  the  more  recent   publications  of  E.   HATSCHEK  in  the 

Kolloid-Zeitschrift . 


108  COLLOID   CHEMISTRY 

between  the  materials  necessary  for  the  formation  of  the 
precipitates  are  maintained.1 

Fig.  38  illustrates  another  interesting  property  of  gels. 
When  gelatin  is  painted  upon  a  glass  plate  and  is  then 
allowed  to  freeze,  the  water  in  the  gel  crystallizes,  under 
proper  experimental  conditions,2  to  form  the  well-known 
frost  figures.  In  the  process,  the  water  naturally  separates 
from  the  gelatin  or  pushes  it  aside.  If  the  cooled  plate  is 
now  thawed,  the  ice  crystals  disappear  but  the  gel  maintains 
the  shape  given  it  by  the  frost  figures.  In  this  way  negatives 
of  the  ice  crystals  may  be  obtained  which  may  be  lasting 
and  which  at  times  are  remarkably  pretty.  The  illustration 
shown  in  Fig.  38  as  well  as  the  method  of  producing  such 
frost  pictures  is  the  work  of  R.  E.  LIESEGANG. 

Finally,  I  wish  to  show  you  a  third  set  of  preparations 
which  will  serve  to  demonstrate  to  you  the  tremendous 
energy  changes  incident  to  the  changes  in  state  of  gels. 
When  a  gelatin  solution  is  dried  upon  a  glass  plate,  as  in  an 
oven  at  100°  C.,  the  gel  contracts.  But  at  the  same  time  it 
sticks  so  fast  to  the  glass  that  large  shell-shaped  pieces  are 
torn  off  the  surface  of  the  glass  as  shown  in  Fig.  39.  I 
have  been  told  that  the  method  is  used  technically  in  the 
manufacture  of  certain  types  of  opaque  window  glass. 

§16. 

I  need  to  hasten  on  to  the  consideration  of  some  further 
changes  in  the  state  of  colloid  systems.  Since  the  earliest 
days  of  colloid  chemistry,  many  studies  have  been  made  of 

1  Instructions  originating  with  R.  E.  LIESEGANG  which  I  have  found  to 
give  splendid  results  read  as  follows:    a  gel  is  prepared  from  4  grams  of  gel- 
atin, 120  grams  of  water  and  0.12  gram  potassium  bichromate.     The  silver 
nitrate  solution  contains  8.5  grams  of  the  solid  salt  in  100  cc.  of  water. 

2  Following  the  method  of  R.   E.   LIESEGANG    (Prometheus,   25,    369), 
glass  plates  are  covered  with  a  thin  layer  of  a  2  to  10  percent  gelatin  solu- 
tion and  before  these  have  dried  they  are  exposed  to  cold.     In  order  to  ob- 
tain the  best  pictures,  it  is  well  to  keep  the  preparations  for  some  time  in 
the    cold.     The   ice  evaporates  after   about   a   day.     The   gelatin  lamellse 
dried  in  this  fashion  then  maintain  their  structure  even  if  after  this  treat- 
ment they  are  brought  back  to  room  temperature. 


THE  CHANGES  IN  STATE  OF  COLLOIDS 


109 


FIG.  38.  — Artificial  frost  pictures  in  gelatin. 


110  COLLOID  CHEMISTRY 

the  coagulation  of  colloids;  in  other  words,  of  the  decreases 
in  degree  of  dispersion  which  lead  to  the  formation  of  micro- 
scopic and  macroscopic  dispersoids.  It  may  be  said  fairly 
that  our  so-called  theories  of  the  colloid  state,  as  proposed 
from  time  to  time,  have  all  centered  about  the  explanations 
that  they  have  offered  of  the  process  of  coagulation.  There 
have  been  proposed  electrical,  chemical  and  mechanical 
theories  of  the  colloid  state  by  which  were  usually  meant 
corresponding  theories  of  the  process  of  coagulation.  Each 
of  these  attempted  to  make  one  of  these  principles  either 
the  only  factor  or  at  least  the  most  important  factor  in  the 
process  of  coagulation. 

In  considering  the  forces  that  are  active  in  the  destruction 
of  the  colloid  state  through  coagulation,  I  would  like  to  have 
you  recall  what  was  said  in  our  first  lecture  regarding  the 
production  of  colloid  systems.  We  discovered  then  many 
methods  and  many  types  of  energy  by  which  we  could  bring 
about  a  change  in  the  degree  of  dispersion.  We  may,  in 
consequence,  expect  a  similarly  great  number  to  be  active 
in  the  induction  of  coagulation. 

We  need  not,  therefore,  expect  to  find  of  dominant  im- 
portance some  one  method  of  coagulation,  as  we  thought  for- 
merly, but  a  whole  series  of  different  although  coordinated  ones. 
As  a  matter  of  fact,  any  attempt  to  classify  the  different 
methods  of  inducing  coagulation  shows  present  in  all  of  them 
a  mixture  of  mechanical,  electrical  and  chemical  forces, 
which  together  lead  to  the  radical  decrease  in  degree  of 
dispersion  that  we  term  coagulation. 

To  enter  for  a  moment  into  details,  we  observe  that  in 
the  coagulation  of  suspensoids  through  electrolytes,  electrical 
phenomena  play  a  chief  role.  Disperse  particles  having 
opposite  charges  are  particularly  liable  to  precipitate  each 
other.  Thus  the  negatively  charged  metal  sols  are  pre- 
cipitated through  very  low  concentrations  of  acids,  that  is, 
by  positively  charged  hydrogen  ions.  Other  cations,  like 
those  of  the  neutral  salts,  act  similarly.  On  the  other  hand 
a  positively  charged  sol  like  that  of  iron  hydroxid  is  precip- 


THE   CHANGES   IN   STATE  OF  COLLOIDS 


111 


itated  most  easily  by  bases,  in  other  words,  by  negatively 
charged  hydroxyl  ions.  In  the  case  of  this  colloid  the  anions 
of  the  neutral  salts  are  particularly  effective  in  producing 
precipitation.  The  great  influence  of  the  electrical  factor 
in  all  these  coagulations  is  strikingly  evidenced  by  the 


FIG.  39.  —  Chipping  of  a  glass  plate  brought  about  by  drying  the  gelatin. 

importance  of  the  valence  of  the  precipitating  ions.  The 
coagulating  power  of  different  salts  for  a  gold  sol  increases 
in  the  order: 

NaCl        MgCl2        Aids 
while  for  iron  hydroxid  it  increases  in  the  order: 

NaCl        Na2SO4       Na3(C6H5O7). 

Much  smaller  quantities  of  the  salts  yielding  ions  of  high 
valence  are  necessary,  therefore,  to  bring  about  a  radical 
decrease  in  degree  of  dispersion,  than  of  those  yielding  ions 
of  lower  value. 

Oppositely  charged  colloids  precipitate  each  other;    thus 


112  COLLOID  CHEMISTRY 

colloid  gold  precipitates  colloid  iron  hydroxid,  and  congo 
red  precipitates  aluminium  hydroxid  (demonstration).  Pre- 
cipitation in  these  instances  is  apparently  brought  about 
through  electrostatic  attraction  of  the  particles,  neutraliza- 
tion of  their  charges  and  coalescence.  It  is  interesting  to 
note  how  very  low  may  be  the  concentrations  of  electrolytes 
which  suffice  to  bring  about  a  precipitation  of  suspensoids. 
Ordinary  india  ink,  for  example,  may  not  be  diluted  with 
tap  water,  for  the  salts  contained  in  this  water  suffice  to 
precipitate  the  ink. 

Coagulation  of  hydrated  emulsoids  through  electrolytes 
represents  a  much  more  complex  problem.  We  expect  this 
for  we  know  that  in  the  precipitation  of  a  protein  solution 
through  neutral  salts,  two  changes  must  occur  side  by  side. 
There  must  first  occur  a  dehydration  (which  in  itself  leads 
to  an  increase  in  degree  of  dispersion),  and  then  a  coincident, 
or  subsequent  coalescence  of  the  particles  into  coarsely 
dispersed  aggregates.  The  importance  of  the  hydration 
element  is  strikingly  evidenced  by  the  fact  that  in  order  to 
Tender  precipitation  possible  relatively  large  amounts  of 
salt  are  required.  Hydrated  emulsoids  are  for  this  reason 
more  stabile  than  suspensoids,  which  we  found  above  so  very 
sensitive  to  low  concentrations  of  different  salts.  The 
dehydrating  effects  of  neutral  salts,  as  well  as  the  dehydrat- 
ing effects  of  alcohol,  etc.,  are  probably  not  primarily 
electrical  in  nature,  even  though  the  electrical  charge1  of  the 
substances  used  may  not  be  without  influence.  The  de- 
hydrating agents  follow  their  own  physico-chemical  laws, 
which,  however,  are  still  but  little  understood.  Electrical 
and  non-electrical  processes  are,  therefore,  associated  in 
the  precipitation  of  hydrated  emulsoids  through  neutral 
salts.  Perhaps  the  matter  is  best  illustrated  in  the  studies 
of  W.  PAULI,  which  show  the  whole  salt,  in  other  words,  both 
ions,  to  play  an  important  part  in  coagulation.  The  effects 

1  See  particularly  the  papers  of  WOLFGANG  PAULI  and  his  pupils  in  the 
Biochemische  Zeitschrift,  the  Kolloid-Zeitschrift  and  the  Kolloid-chemische 
Beihefte.  See  for  example  WOLFGANG  PAULI,  Koll.-Zeitschr.,  7,  241  (1910); 
12,  222  (1913);  H.  HANPOVSKY,  Koll.-Zeitschr.,  7,  183,  267  (1910). 


THE  CHANGES  IN  STATE  OF  COLLOIDS  113 

of  any  salt  represent  the  algebraic  sum  of  its  constituent 
ions,  the  effects  of  which  may  be  additive  or  antagonistic. 
The  non-electrical  nature  of  dehydrating  and  coagulating 
effects  is  most  clearly  to  be  observed  in  electrically  neutral 
or  but  weakly  charged  emulsoids,  while  in  the  heavily 
charged,  such  as  acid  or  alkali  proteins,  the  electrical  effects 
again  come  to  the  front.  In  the  precipitation  of  acidified 
(positively  charged)  proteins,  the  coagulating  power  of  the 
neutral  salts  follows  the  well-known  HOFMEISTER  series. 
Anions  follow  the  order: 

chlorate,  nitrate,  chlorid,  acetate,  sulphate,  tartrate; 

the  bases,  the  order: 

magnesium,  ammonium,  sodium,  potassium,  lithium. 

These  series  are  reversed  when  alkalinized  protein  instead  of 
acid  protein  is  used.  The  important  role  of  the  valence  of 
the  ions  largely  disappears  as  soon  as  neutral  albumin  is  used, 
though  this  is  still  capable  of  being  precipitated  by  neutral 
salts,  according  to  the  experiments  of  W.  PAULI.  These 
facts  prove  that  in  this  case  the  electrical  relationships  play 
only  a  secondary  role  and  that  this  type  of  coagulation  may 
perhaps  best  be  designated  as  coagulation  through  withdrawal 
of  solvent. 

§17. 

Interesting  phenomena  are  observed  when  the  coagulation 
of  mixtures  of  suspensoids  with  hydrated  emulsoids  is 
studied.  The  greater  stability  of  the  emulsoid  fraction  is 
then  carried  over  to  the  suspensoid  fraction  which,  in  con- 
sequence, becomes  less  sensitive  to  concentrations  of  salt 
which  previously  coagulated  it.  We  say  the  emulsoids  exer- 
cise a  protective  action  upon  the  suspensoids  and  explain  the 
phenomenon  by  assuming  that  the  fluid  emulsoid  droplets 
surround  the  suspensoid  particles,  or  in  some  way  combine 
with  them — a  view  well  supported,  for  example,  by  ultrami- 
croscopic  observations.  The  protective  action  is  shown  even 
by  traces  of  emulsoids,  another  fact  in  harmony  with  the 


114  COLLOID  CHEMISTRY 

explanation  just  given,  for  theoretically  only  very  little  emul- 
soid  material  is  necessary  to  surround  a  suspensoid  particle. 
Much  use  is  made  in  scientific  and  technical  colloid 
chemistry  of  this  protective  action  of  the  emulsoids.  Thus, 
the  presence  of  traces  of  gelatin  enables  us  to  produce 
more  concentrated  and  more  highly  dispersed  sols  than 
can  be  produced  in  pure  dispersing  media  alone.  The 
effect  of  tannin  in  the  production  of  a  highly  dispersed 
red  gold  of  the  type  I  showed  you  in  my  first  lecture  de- 
pends in  good  part  upon  such  a  protective  action.  The 
tannin  not  only  acts  as  a  reducing  substance,  but  at  the 
same  time  as  a  protective  colloid.  The  presence  of  a  pro- 
tecting colloid  is  also  of  advantage  because  it  makes  pos- 
sible the  evaporation  of  suspensoid  sols  to  dryness.  Because 
of  the  spontaneous  solubility  of  the  protective  colloid  the 
dried  material,  when  thrown  into  its  solvent,  can  be  brought 
back  into  a  state  of  colloid  solution,  the  suspensoid  fraction 
retaining  under  these  circumstances  its  original  high  disper- 
sion value.  I  show  you  here  a  number  of  such  "  dried 
hydrosols"  (demonstration).  They  are,  as  you  see,  mostly 
dark  colored  scales  which  are  readily  soluble  in  water 
(demonstration) . 

§18. 

In  addition  to  the  methods  of  coagulation  brought  about 
through  the  addition  of  different  substances,  others  are 
known  in  which  radiant  energy,  as  from  radium  emanations, 
is  active.  Coagulation  may  also  be  brought  about  through 
exposure  to  light,  through  shaking  with  charcoal,  with 
Fuller's  earth,  or  with  other  powders,  and  through  the 
addition  of  non-miscible  liquids.  Thus,  many  colloids,  in- 
cluding the  proteins,  may  be  separated  almost  completely 
from  their  dispersion  media  by  being  shaken  for  long 
periods  of  time  with  benzol  or  petroleum.  Moreover, 
when  egg  white  is  beaten  to  a  foam,  a  part  is  regularly 
coagulated  in  the  walls  enclosing  the  air  bubbles. 

A  decrease  in  degree  of  dispersion  to  the  point  of  inducing 


THE  CHANGES  IN  STATE  OF  COLLOIDS  115 

« 

coagulation  can  also  be  brought  about  through  centrifug- 
ing,  etc.  These  belong  to  the  mechanical  methods  of  pro- 
ducing coagulation.  Certain  details  of  the  way  in  which 
these  different  methods  produce  their  effects  will  be  dis- 
cussed later. 

§19. 

The  reverse  of  coagulation,  namely,  peptization,  may  also 
be  brought  about  by  the  most  diverse  chemical,  electrical 
and  mechanical  means.  Coagulated  gels,  for  instance,  may 
again  be  brought  to  a  state  of  colloid  dispersion  through 
treatment  with  weak  acids  or  bases,  as  will  be  shown  in 
the  next  lecture.  Of  special  importance  are  the  peptization 
phenomena  in  which  an  increased  dispersion  is  brought 
about  in  a  gel  through  the  addition  of  traces  of  electrolytes. 
Freshly  precipitated  sulphids  may  thus  be  brought  back 
into  colloid  solution  by  being  treated  with  dilute  hydrogen 
sulphid. 

Let  me  illustrate  peptization  to  you  by  an  experiment 
devised  by  A.  LOTTERMOSER  (demonstration).  These  five 
flasks  all  contain  the  same  amount  of  freshly  precipitated 
silver  iodid.  To  one  has  been  added  water  only.  The 
remaining  four  contain  equal  amounts  of  differently  con- 
centrated potassium  iodid  solutions.  The  concentration  of 
the  potassium  iodid  increases  in  the  order  in  which  I  have 
arranged  these  flasks,  amounting  in  the  last  one  to  a  one- 
fourth  molar  solution.1  You  observe  how  in  the  water 
and  in  the  most  concentrated  potassium  iodid  solution  the 
precipitate  remains  coarsely  dispersed.  The  supernatant 
liquid  in  these  is  relatively  clear.  In  the  remaining  flasks 
this  liquid  is  of  milky  appearance,  particularly  marked  in 
the  middle  one  containing  approximately  a  Tfo  molar 
solution.  In  this  concentration  the  gel  has  changed  to  a 
typical  colloid  solution  of  silver  iodid. 

Such   dispersing   effects   due   to   low   concentrations   of 

1  See  A.  LOTTERMOSER,  KoH.-Zeitschr.,  2,  Supplement  1,  4  (1907);  3, 
31  (1908);  6,  78  (1910);  also,  Zeitschr.  f.  phyaikal.  Chemie,  62,  359  (1908). 


116  COLLOID  CHEMISTRY 

electrolytes  are  frequently  observed,  the  ions  capable,  of 
producing  them  being  known  as  stabilizing,  or  sol-forming 
ones.  Their  effect  is  observable  only  when  the  gel  is  freshly 
precipitated  and  is  possessed  of  a  definite  " mechanical" 
constitution.  By  the  latter  is  meant  that,  in  the  fresh 
gel,  the  colloid  particles  are  merely  agglutinated  and  have 
not  yet  coalesced.  Under  such  circumstances  the  ions  can 
apparently  give  the  " primary"  gel  particles  an  electrical 
charge  leading  to  their  electrostatic  repulsion  which  in 
turn  leads  to  the  re-solution  of  the  gel. 

In  many  cases,  both  in  emulsoids  and  in  suspensoids, 
mere  restitution  of  the  original  conditions  suffices  to  make 
a  precipitated  colloid  go  back  into  solution.  We  say  then 
that  the  colloid  change  is  reversible.  Generally  speaking, 
reversible  coagulations  are  commoner  among  the  emulsoids 
than  among  the  suspensoids.  Still  it  is  wrong  to  consider 
irreversibility  of  precipitation  as  directly  characteristic  of 
the  coagulation  of  suspensoids,  as  is  still  done.  Colloid 
silver,  for  example,  yields  a  coagulum  upon  the  addition 
of  ammonium  citrate  or  ammonium  nitrate  which  is  en- 
tirely reversible.1 

§20. 

In  concluding  this  lecture,  let  us  consider  briefly  that  group 
of  colloid  changes  in  state  which  are  comprised  under  the 
heading  of  adsorption.  In  keeping  with  what  was  said  at 
the  beginning  of  this  lecture,  we  may  regard  adsorption  as 
that  change  in  concentration  which  colloids  and  other  dispersed 
systems  suffer  at  the  surfaces  where  they  come  in  contact  with 
other  bodies.  This  change  in  concentration  is  the  only 
constantly  observed  behavior  that  is  common  to  all  the 
myriad  manifestations  generally  grouped  under  the  term 
adsorption.  We  shall,  therefore,  consider  only  this  phase 
of  the  problem.  After  such  a  concentration  difference  has 
come  to  pass,  a  long  series  of  secondary  changes  may  take 
place. 

1  See  S.  ODEN  and  E.  OHLON,  Zeitschr.  f.  physikal.  Chemie,  82,  78  (1913). 


THE  CHANGES  IN  STATE  OF   COLLOIDS  117 

In  cases  of  positive  adsorption  there  commonly  occurs 
a  fixation  of  the  colloid  or  of  the  dispersed  material  upon 
the  solid,  liquid  or  gaseous  adsorbing  surface.  Under  such 
circumstances,  the  rest  of  the  dispersed  system  may  be 
poured  off  without  carrying  away  with  it  the  dispersed 
material  that  has  collected  in  the  surface.  Such  fixation 
in  the  surface  may  be  brought  about,  for  instance,?  by 
the  colloid  being  coagulated  into  a  coherent  layer.  As  a 
matter  of  fact,  the  mechanical  coagulations  discussed  above 
rest  in  part  upon  such  primary  adsorption  effects.  The 
increase  in  concentration  may  go  so  far  that  the  adsorbed 
material  separates  out  in  solid  or  even  in  crystalline  form 
upon  the  edge  of  the  adsorbent,  as  observable  in  the  ad- 
sorption of  organic  dyes  by  charcoal.  At  other  times  the- 
adsorbed  material  may  wander  into  the  adsorbent  and  there 
form  a  liquid  or  solid  solution.  This  can  occur  of  course 
only  when  the  adsorbed  materials  are  capable  of  diffusion, 
in  other  words,  are  molecular  dispersoids.  The  phenom- 
enon is  illustrated  by  the  adsorption  of  iodin  by  charcoal. 
Finally,  in  consequence  of  an  accumulation  of  the  dis- 
persed particles  in  a  surface,  chemical  reactions  of  various 
kinds,  more  especially  polymerizations,  may  occur.  Such 
are  actually  observed,  for  example,  in  the  adsorption  of 
starches.1  More  pronounced  chemical  reactions  like  hy- 
drolyses,  oxidations,  etc.,  may  also  be  observed.  But  all 
these  changes  are  secondary  and  may  be  of  totally  differ- 
ent types  in  the  different  special  cases  of  adsorption  studied. 
The  primary  and  only  constant  change  which  character- 
izes adsorption  is  found  in  the  change  in  concentration  of 
the  dispersed  material  that  occurs  in  the  surface  layer. 

Before  showing  some  adsorption  experiments,  it  is  im- 
portant to  emphasize  that  the  intensity  of  adsorption  is 
chiefly  dependent  upon  the  size  of  the  adsorbing  surface. 
It  is  for  this  reason  that  for  practical  adsorption  purposes 
we  use  highly  dispersed  powders  of  carbon,  Fuller's  earth, 
etc.  The  enormity  of  the  adsorbing  surfaces  in  such  dis- 
1  See  L.  GURWITSCH,  Koll.-Zeitschr.,  11,  17  (1912). 


118 


COLLOID  CHEMISTRY 


persed  systems  is  not  generally  appreciated,  nor  how 
greatly  these  grow  with  increase  in  the  division  of  the 
particles.  To  illustrate  the  matter  I  append  the  following 
table  which  shows  the  surface  increase  of  one  cubic  centi- 
meter when  undergoing  decimal  division: 

INCREASE  IN  SURFACE  OF  A  CUBE  WHEN  DECIMALLY  DIVIDED. 


Length  of  Cube  Edge. 

Number  of  Cubes. 

Total  Surface. 

1  cm  
1  mm  

1 
103 

6  sq.  cm. 
60 

0  1  mm. 

106 

600 

0  01  mm. 

109 

6  000 

1  n 

1012 

6  so    m 

0.1  M.  • 

1015 

60 

0.01  M  

1018 

600 

1  MM  

1021 

6000 

0.1  MM  
0.01  MM  

1024 
1027 

60,000 
600,000 

0  001  MM 

1030 

6  so    km 

In  this  table  is  shown  how  a  solid  one  centimeter  cube 
of  carbon  acquires  a  total  surface  of  60  square  meters  when 
divided  to  the  point  of  microscopic  visibility  (0.1  /*),  and  one 
of  60  to  600  square  meters  if  the  subdivision  is  carried 
through  the  colloid  realm.  A  sugar  manufacturer  buying 
a  cubic  meter  of  charcoal  for  clarifying  purposes  receives 
when  this  consists  of  particles  one  millimeter  in  diameter, 
some  600  square  meters  of  surface;  while  if  the  particles 
are  1  /x  in  diameter  he  receives  six  million  square  meters, 
or  six  square  kilometers  of  adsorbing  surface  for  his  money. 
These  figures  illustrate  to  what  extraordinary  values  these 
surface  increases  mount  when  dealing  with  highly  dispersed 
adsorbents. 

I  want  now  to  show  you  the  wide  distribution  of  these 
adsorption  phenomena  (demonstration).  I  have  here ^ a 
number  of  flasks  filled  with  differently  colored  colloid  and 
molecularly  dispersed  liquids;  picric  acid,  iron  chlorid, 
fuchsin,  berlin  blue,  congo  red,  colloid  silver,  colloid  gold 
and  colloid  graphite.  Standing  opposite  is  an  equal  number 


THE  CHANGES  IN  STATE  OF  COLLOIDS  119 

of  flasks,  each  containing  a  spoonful  or  two  of  bone  char- 
coal (bone  black).  I  pour  upon  the  bone  black  in  each  of 
the  flasks  the  contents  of  the  flask  opposite  it  and  for  a 
moment  shake  every  mixture  vigorously.  I  next  pour  all 
the  mixtures,  along  with  their  bone  black,  one  after  the 
other  into  this  large  filter-lined  funnel.  In  spite  of  the 
fact  that  I  have  carried  out  this  whole  procedure  as  rapidly 
as  possible,  you  see  that  the  filtrate  runs  through  entirely 
clear.  Adsorption  has,  in  other  words,  occurred  with 
extraordinary  rapidity  in  all  these  cases  and  is  practically 
complete. 

To  demonstrate  that  not  only  colored  dispersoids  and 
not  only  charcoal  show  such  prompt  adsorptive  effects,  I 
show  you  an  experiment  with  a  colorless  alkaloid,  quinin 
bisulphate,  and  a  special  preparation  of  Fuller's  earth,  pre- 
pared by  JOHN  URI  LLOYD  of  Cincinnati;1  10  cc.  of  the 
clear  quinin  solution  are  poured  upon  a  half  gram  of  the 
dry  adsorbent,  the  flask  is  shaken  and  the  mixture  allowed 
to  stand  a  moment.  To  prove  to  you  that  the  original 
quinin  solution  was  rich  in  this  alkaloid  —  it  was  a  2.5  per- 
cent solution  —  I  acidify  it  slightly  and  add  MAYER'S 
alkaloid  reagent  to  it  (demonstration).  A  heavy  white 
precipitate  forms  at  once.  Let  me  now  filter  the  adsorp- 
tion mixture  and  repeat  this  test  with  the  filtrate.  One 
hardly  expects,  of  course,  that  adsorption  will  be  complete 
and  that  our  filtrate  will  not  show  some  slight  turbidness, 
and  yet  as  I  perform  the  test  you  observe  that  the  filtrate 
remains  absolutely  clear  no  matter  how  much  reagent  or 
acid  I  add.2 

1  I  am  much  indebted  to  Professor  J.  U.  LLOYD  for  giving  me  the  ma- 
terial necessary  for  this  demonstration.     The  preparation  appears  in  the 
trade  under  the  name  of  LLOYD'S  Alkaloidal  Reagent. 

2  According   to   the   accompanying    circular    the    following   amounts   of 
LLOYD'S  reagent  are  required.     To  adsorb  completely  one  gram  of  cocain- 
hyarochlorid,  there  are  required  of  the  reagent  10  grams;    for  one  gram  of 
strychnin  sulphate,  10  grams  of  the  reagent;    for  one  gram  cinchonin  sul- 
phate,  10  grams;    for  one  gram  cinchonidin  sulphate,   10  grams;    for  one 
gram  quinin  bisulphate,  8  grams;    for  one  gram  atropin  sulphate,  8  grams; 
for  one  gram  brucin  sulphate,  5.6  grams;    for  one  grain  codein  sulphate. 


120 


COLLOID  CHEMISTRY 


When  specific  instances  of  adsorption  are  studied  we 
observe  considerable  differences  so  far  as  intensity  of 
adsorbing  power  is  concerned.  Acids,  for  example,  are 
usually  better  adsorbed  than  their  salts;  and  organic  salts, 
better  than  inorganic.  Substances  of  high  molecular 
weight  and  colloids  are  especially  well  adsorbed.  The 
ready  adsorption  of  the  latter  has  been  regarded  by  many 
authors  as  specific  of  them.  Of  course  there  exist  quanti- 
tative differences  among  these,  too.  If,  in  my  previous 
experiment,  I  had  chosen  to  mix  a  concentrated  arsenious 
trisulphid  with  bone  black  and  had  poured  this  mixture 
upon  the  filter,  I  might  not  have  been  so  successful  in  get- 
ting an  absolutely  clear  filtrate. 

Of  much  importance  are  certain  quantitative  relations 
as  observed  when  adsorption  is  allowed  to  occur  from 
solutions  of  different  concentrations.  Generally  speaking, 
adsorbents  take  up  relatively  more  from  dilute  solutions 

than  from  more  concen- 
trated ones.  In  most 
instances  an  adsorption 
maximum  is  attained  be- 
yond which  no  increased 
amounts  of  the  adsorbed 
material  are  taken  up  by 
the  surface  of  the  adsorb- 
ent. The  "  concentration 

FIG.  40.  —  Diagram     illustrating     the    function"1    of    adsorption 
concentration  function  in  adsorption,     when  graphically  expressed 

in  solution  and  in  chemical  combi-   has,  therefore,  a  hyperbolic 

form  as  shown  in  Fig.  40, 

A.      This    concentration    function    of    adsorption    differs 
markedly  from  the  concentration  function  expressing  the 

5  grams;   for  one  gram  morphin  sulphate,  4  grams.     The  mixtures  are  usu- 
ally prepared  in  from  100  to  200  cc.  of  water. 

1  In  modern  literature  we  frequently  encounter  the  term  adsorption 
isotherm  as  a  designation  for  this  concentration  function.  The  use  of  thia 
term  represents  a  by  no  means  justified  coquetry  with  the  simpler  systems 
of  physical  chemistry  containing  but  three  variables  (as  is  the  case,  for 


THE  CHANGES  IN   STATE  OF  COLLOIDS  121 

distribution  of  a  dissolved  substance  between  two  non- 
miscible  liquids.  The  distribution  of  a  salt,  for  instance,  be- 
tween water  and  chloroform  is  represented  by  a  straight  line 
as  shown  by  B  in  Fig.  40.  For  purposes  of  comparison,  we 
may  introduce  the  effects  of  the  formation  of  stoichiometrical 
combinations  of  a  chemical  nature  between  adsorbent  and 
adsorbed  material.  The  two  molecules  reacting  with, each 
other  might  under  these  circumstances  be  regarded  as 
adsorbent  and  material  to  be  adsorbed,  the  adsorbed 
amount  being  calculated  as  number  of  molecules  bound. 
Saturation  'of  all  the  molecules  considered  as  adsorbent 
by  the  material  to  be  adsorbed  would  under  such  circum- 
staifces  not  be  attained  until  a  definite  stoichiometrical 
concentration  of  the  latter  had  been  reached.  At  this 
point  the  number  of  the  molecules  constituting  the  ad- 
sorbent would  be  just  enough  to  yield  a  definite  chemical 
compound.  But  at  this  concentration  all  the  molecules 
of  the  material  to  be  adsorbed  would  have  been  used  up 
and  the  composition  of  the  reaction  product  would  then 
not  change  even  when  an  excess  of  the  molecules  being 
adsorbed  was  added.  The  whole  process  represented 
graphically  would  yield  therefore  a  right  angled  curve,  as 
shown  in  C  of  Fig.  40.  All  possible  transitional  types 
are  discoverable  between  these  three  curves.  Some  of 
them  are  extremely  interesting  but  their  detailed  consider- 
ation would  lead  us  too  far  afield. 

§21. 

If  it  is  asked  what  forces  bring  about  these  concentration 
changes  in  surfaces,  it  can  only  be  answered  that  a  whole 
series  of  different  kinds  of  energy  plays  a  role.  That  which 

example,  in  gases)  in  which,  through  exclusion  of  the  one  variable,  the  func- 
tion of  the  remaining  two  is  then  easily  obtainable.  But  in  adsorption 
equilibria  this  does  not  hold  true,  since  at  constant  temperature  one  may, 
for  example,  obtain  a  whole  series  of  adsorption  isotherms,  dependent  upon 
variations  in  the  amount  of  absorbing  material  or  variations  in  its  specific 
surface.  To  speak  of  an  "isotherm"  under  such  circumstances  is  to  invite 
objection.  The  expression  "concentration  function"  is  much  to  be  preferred. 


122  COLLOID  CHEMISTRY 

*  is  common  to  all  the  forces  active  in  adsorption  is  expressed 
in  a  generalization  of  WILLARD  GIBBS'  theorem,  and  reads 
as  follows:  adsorption  will  take  place  whenever  there  exists  in 
a  surface  a  difference  in  energy  potential  which  can  be  de- 
creased through  a  change  in  the  concentration  of  the  dispersed 
materials  bordering  upon  this  surface.1 

To  get  a  picture  of  what  this  means,  imagine  a  solid 
dipping  into  a  liquid  toward  which  the  solid  possesses  an 
electrical  charge.  There  exists  in  the  surface,  in  this  case, 
an  electrical  difference  in  potential.  If  a  dispersed  phase 
carrying  a  charge  opposite  to  that  possessed  by  the  solid 
body  is  present  in  the  liquid,  the  difference  in  electrical 
potential  can  evidently  be  decreased  by  having  the  dis- 
persed particles  aggregate  in  the  surface  and  so  partially 
neutralize  it.  The  consequence  would  be  an  electrical 
adsorption  effect. 

Next  imagine  two  substances,  such  as  two  non-miscible 
liquids,  between  which  there  plays  the  ordinary  surface 
tension.  If  the  one  liquid  is  a  dispersoid  whose  surface 
tension  against  the  second  liquid  decreases  with  increase 
in  concentration,  there  will  also  be  a  tendency  toward 
positive  adsorption,  the  adsorption  being  this  time  mechani- 
cal in  nature.  This  type  of  adsorption  consequent  upon 
decreases  in  surface  tension  was  recognized  by  WILLARD 
GIBBS  and  J.  J.  THOMSON,  and  has  in  recent  years  been 
the  subject  of  much  study.  Its  importance  seems  actually 
to  have  been  overestimated,  for  even  recent  writers  have 
declared  it  to  be  the  only  possible,  or  at  least  the  only 
universally,  present  factor  in  all  adsorptions. 

Imagine,  finally,  that  there  exists  between  two  phases, 
as  between  an  adsorbent  and  a  dispersoid,  a  chemical  dif- 
ference in  potential.  A  chemical  reaction  is  proceeding 
in  the  surface  which,  like  most  chemical  reactions,  increases 
in  rate  with  increase  in  the  concentration  of  the  substances 
concerned  in  the  reaction.  Under  such  circumstances  there 

1  This  extension  of  GIBE'S  rule  was  made  in  my  Grundriss  der  Kolloid- 
chemie,  1.  Ann.,  434,  Dresden,  1909. 


THE  CHANGES  IN  STATE   OF  COLLOIDS  123 

would  also  appear  a  tendency  of  the  dispersed  phase  to 
accumulate  in  the  surface,  thus  yielding  an  adsorption  in 
which  the  driving  force  would  be  chemical  in  nature. 

Similar  reasoning  governs  the  effects  of  thermic  and 
photic  differences  in  potential  in  surfaces.  A  whole  series 
of  different  energies,  in  other  words,  plays  a  role  in  adsorp- 
tion, and  these  not  only  may  be  coordinated  with  each 
other,  but  may  actually  at  times  antagonize  each  other.1 
These  facts  will  suffice  to  emphasize  the  importance  of 
distinguishing  in  any  individual  case  between  the  different 
kinds  of  energy  that  may  be  playing  a  part  in  the  produc- 
tion of  the  adsorption. 

§22. 

Permit  me,  in  conclusion,  to  touch  upon  the  remarkable 
and  interesting  illustrations  of  adsorption  that  are  seen  in 
the  mutual  adsorption  of  two  colloids  or  in  the  mutual  ad- 
sorption of  a  molecular  dispersoid  and  a  colloid.  Which 
in  either  illustration  is  the  adsorbent  and  which  the  ad- 
sorbed material?  Obviously  in  mutual  adsorptions  of 
highly  dispersed  systems  all  differences  between  adsorbent 
and  adsorbed  material  disappear,  just  as  in  the  union  of 
two  molecules  with  each  other  chemically.  But  this  anal- 
ogy to  the  " purely  chemical"  reaction  goes  further.  In 
the  mutual  adsorption  of  two  dispersed  phases,  the  first 
change  that  occurs  consists  in  a  decrease  in  the  degree  of 
dispersion  of  the  whole  system.  This  is  illustrated  in  the 
mutual  precipitation  of  two  oppositely  charged  colloids. 
But  this  is  like  the  formation  of  a  precipitate  in  any  chem- 
ical reaction  for  which  there  is  also  necessary  as  a  first 
change  the  mutual  adsorption  of  two  dispersed  systems. 
Further,  experiment  has  shown  that  the  precipitation  of 
colloids  often  necessitates  the  existence  of  the  reacting 

1  Such  antagonistic  and  complex  adsorption  processes  occur,  for  example, 
when  electrical  and  surface-tension  differences  appear  simultaneously  in 
surfaces.  See  my  Grundriss  der  Kolloidchemio,  1.  Aufl.,  435,  Dresden,  1909. 


124  COLLOID   CHEMISTRY 

materials  in  definite  concentrations.1  The  quantitative  re- 
lationships obtaining  in  the  reaction  mixture  may  actually 
yield  stoichiometrical  values.2  These  facts  further  demon- 
strate  the  analogy  between  adsorption  and  the  chemical 
union  of  molecules  and  show  how  difficult  it  may  be  under 
some  circumstances  to  determine  whether  the  formation 
of  a  precipitate  has  been  occasioned  by  purely  chemical 
means  or  by  those  associated  with  the  physical  conse- 
quences of  adsorption.  Finally,  if  it  is  recalled  that  adsorp- 
tion phenomena  are  known  which  are  dependent  upon 
certain  chemical  relationships,  we  become  aware  of  the 
bridge  which  exists  between  colloid-chemical  reactions  and 
the  purely  chemical  ones.  The  relationship  between  the 
two  appears  to  be  so  close  that  its  consequences,  when 
applied  to  our  present  " chemical"  notions,  seem  nothing 
short  of  startling. 

§23. 

I  cannot  say  that  with  these  remarks  and  with  these 
experiments  on  theoretical  colloid  chemistry,  I  have  con- 
cluded the  subject,  though  necessity  compels  me  to  break 
off  here.  I  can  only  hope  to  have  made  you  realize  the 
wealth  of  material  that  this  new  science  possesses.  Had 
I  had  at  my  disposal  twice  the  time  allotted  me,  I  could 
still  only  have  given  you  a  sketch  of  the  field.  In  the  two 
lectures  that  follow  I  shall  try  to  give  you  a  glimpse  of  the 
scientific  and  technical  applications  that  may  be  made  of 
colloid  chemistry. 

1  See  especially  the  papers  of  J.  M.  VAN  BEMMELEN,  W.  BILTZ,  etc.     Ref- 
erences may  be  found  in  my  Grundriss  der  Kolloidchemie,  1.  AufL,  404, 
Dresden,  1909. 

2  See,  for  example,  A.  SANIN,  Koll.-Zeitschr.,  13,  305  (1913). 


IV. 

SOME  SCIENTIFIC  APPLICATIONS  OF  COL- 
LOID  CHEMISTRY. 


FOURTH  LECTURE. 

SOME  SCIENTIFIC  APPLICATIONS  OF  COL- 
LOID CHEMISTRY. 

WE  shall  devote  these  last  two  lectures  to  the  applications 
that  may  be  made  of  colloid  chemistry.  Applications  may 
be  made  of  a  science  under  two  headings.  One  science  may 
be  applied  to  a  second,  for  example.  Not  only  is  this 
possible  but  it  must  be  done  in  certain  instances  as  when  the 
principles  of  physics  or  of  chemistry  are  applied  to  biology 
or  mineralogy.  We  cannot,  of  course,  apply  haphazard  any 
given  science  to  some  other.  While  chemistry,  for  example, 
may  be  applied  to  the  biological  problem  of  heredity,  the 
converse  cannot  be  done,  though  into  the  philosophical 
reasons  for  this  we  cannot  enter  here.1  Second,  we  may 
apply  science  to  technical,  practical  and  industrial  problems. 

Colloid  chemistry  also  finds  application  in  these  two  ways. 
In  fact  I  cannot  begin  a  discussion  of  this  question  without 
declaring  that  since  the  birth  of  the  so-called  classical  physical 
chemistry  of  the  molecular  solutions,  some  thirty  years  ago,  no 
branch  of  physics  or  chemistry  has  arisen  which  can  be  com- 
pared in  importance,  so  far  as  scientific  and  technical  applica- 
tions are  concerned,  with  that  of  colloid  chemistry.  I  am 
fully  aware  of  the  magnitude  of  this  claim,  yet,  as  this  and 
the  next  lecture  will  show,  I  stand  ready  to  defend  this 
thesis.  I  know  very  well,  for  example,  that  radio-chemistry, 
which  in  point  of  age  may  be  regarded  as  a  sister  science  to 
colloid  chemistry,  has  yielded  results  which  have  modified 
most  drastically  and  broadened  in  surprising  fashion  our 
concept  of  nature;  but  so  far  as  the  applications  are  con- 
cerned that  may  be  made  of  it,  either  in  point  of  number  or 

1  The  application  of  one  science  to  another  is  regulated  by  the  relations 
expressed  in  the  well-known  pyramid  of  the  sciences  of  A.  COMTE  and 

WlLHELM   OSTWALD. 

127 


128  COLLOID  CHEMISTRY 

in  variety,  to  scientific  and  technical  problems,  even  radio- 
chemistry  cannot  compare  with  colloid  chemistry. 

§1. 

If  I  have  previously  complained  that  the  wealth  of 
phenomena  and  ideas  embraced  in  pure  colloid  chemistry 
was  too  great  to  permit  me  to  give  you  a  proper  conception 
of  it,  I  can  only  complain  more  loudly  when  I  am  asked  to 
outline  the  applications  that  may  be  made  of  this  science. 
I  do  not  believe  that  anyone  would  even  today  essay  to  read 
all  the  papers  that  have  been  written,  for  example,  on  the 
applications  of  colloid  chemistry  to  biological  and  medical 
problems.  Whole  books  are  required  to  give  you  the 
colloid-chemical  views  that  are  of  such  paramount  impor- 
tance in  even  such  special  and  technical  fields  as  those  of 
dyeing  and  tanning.  It  is  no  exaggeration  to  say  that  no 
week  has  gone  by  in  the  past  ten  years  (and  of  course  does 
not  go  by  today)  in  which  a  new  body  of  scientific  or 
technical  facts  is  not  recognized  as  essentially  colloid- 
chemical  in  nature,  and  in  which  it  is  not  shown  how  the 
application  of  colloid  chemistry  to  such  phenomena  not  only 
brings  immediate  light  but  promise  of  even  more  in  the 
future.  In  such  development  over-enthusiasm  of  course 
will  sometimes  evidence  itself  and  then  problems  will  be 
designated  as  colloid-chemical  which  in  reality  belong  in  a 
different  category  of  science.  But  in  spite  of  such  occasional 
lapses,  everyone  who  knows  the  history  of  applied  colloid 
chemistry  will  agree  not  only  hi  admitting  that  the  number 
and  the  variety  of  the  problems  to  which  colloid  chemistry 
is  applicable  is  already  amazingly  large,  but  that  the  ulti- 
mately attainable  by  such  application  can  at  present  hardly 
be  imagined.  Speculation  in  colloid-chemical  futures  is 
still  entirely  safe. 

In  discussing  with  you  the  applications  of  colloid  chemistry 
I  must,  because  of  limited  time,  take  my  choice  between 
presenting  a  few  illustrations  in  some  detail,  or  a  larger 
number  more  superficially.  I  shall  follow  the  second  course 


SCIENTIFIC  APPLICATIONS  129 

which,  though  less  satisfactory  in  some  respects,  will  best 
serve  to  emphasize  for  you  the  varieties  of  colloid-chemical 
application. 

§2. 

When  we  take  up  the  scientific  applications  of  colloid 
chemistry,  we  recognize  at  once  that  a  great  number  of  such 
may  be  made  within  the  field  of  chemistry  itself.  We  need 
but  recall  our  first  lecture  to  recognize  how  a  discussion  of 
precipitates  and  of  their  properties  of  passing  through  filter 
paper  at  once  plunges  us  into  the  midst  of  that  branch  of 
chemistry  in  which  we  received  our  first  instruction,  namely, 
analytical  chemistry.  The  rules  laid  down  by  analytical 
chemistry  with  an  eye  to  avoiding  the  " going  through"  of 
a  precipitate,  such  as  the  working  with  relatively  concen- 
trated solutions,  the  setting  aside  of  the  precipitate,  moderate 
heating,  the  addition  of  salts,  etc.,  are  all  of  them  methods, 
as  you  see,  of  utilizing  the  influences  of  concentration,  of 
ageing,  of  coagulation  and  of  the  effects  of  salts  in  deter- 
mining the  degree  of  dispersion  of  colloids  and  their  pre- 
cipitation. 

Another  interesting  application  of  colloid  to  analytical 
chemistry  is  seen  in  the  methods  employed  for  recognizing 
traces  of  the  noble  metals.  In  discussing  the  colors  of  colloids, 
I  called  your  attention  to  the  great  intensity  of  those  shown 
by  the  noble  metals  when  these  are  colloidally  dispersed. 
At  times  it  may  even  exceed  that  of  the  aniline  dyes.  It  is 
natural  that  this  property  of  the  noble  metals  should  have 
been  called  upon  for  analytical  purposes,  and  so  it  does  not 
surprise  us  that  one  of  the  oldest  and  best  known  methods 
of  demonstrating  the  presence  of  traces  of  gold  consists  in 
reducing  this  to  colloid  form.  The  Cassius  purple  test  for 
gold  is  a  typical  illustration  of  the  production  of  gold  in 
colloid  form  and  its  subsequent  precipitation  in  the  form  of 
an  adsorption  compound  through  a  second  colloid.  The 
first  step  in  the  test  is  accomplished  through  the  reduction 
of  the  gold  salt  by  stannous  chlorid.  In  this  way  colloid 


130  COLLOID   CHEMISTRY 

gold  and  colloid  stannous  acid  are  produced,  which  then 
unite  to  form  the  well-known,  reddish-violet  precipitate. 

We  are  familiar  with  still  more  sensitive  colloid-chemical 
methods  of  demonstrating  minute  traces  of  the  noble  metals 
of  which  I  should  like  to  show  you  one  (demonstration). 
As  shown  by  the  far  too  little  known  and  appreciated  in- 
vestigations of  J.  DON  ATI,  dilute  solutions  of  the  noble  metals 
are  reduced  in  our  common  borax  beads  to  solid  colloid 
solutions.  In  this  way  different  colors  are  imparted  to  the 
otherwise  colorless  bead,  which  differ  with  the  character  of 
the  metal  and  its  degree  of  dispersion.  Let  me  fill  this 
platinum  loop  with  some  powdered  borax  and  then  heat  it 
in  a  Bunsen  flame.  When  the  salt  begins  to  "foam,"  I 
moisten  it  carefully  with  a  very  dilute  solution  of  gold 
chlorid  and  then,  by  further  application  of  heat,  melt  the 
whole  mixture  to  a  bead.  As  you  observe,  the  bead  assumes 
the  rose  color  familiar  to  you  as  characteristic  of  highly 
dispersed  gold.  Were  I  to  use  a  more  concentrated  gold 
chlorid  I  should  obtain  a  violet  or  blue  bead.  These 
correspond,  as  you  know,  with  less  highly  dispersed  grades 
of  reduced  gold.  Even  the  red  bead  will,  with  prolonged 
heating,  usually  turn  violet,  for  under  such  circumstances 
the  dispersion  value  of  the  gold  is  progressively  decreased 
until  coagulation  occurs. 

With  silver  salts  one  can  obtain  blue  or  yellow  beads; 
while  platinum  salts  yield  violet  ones.  The  great  sensitive- 
ness of  these  color  reactions  is  of  much  interest.  Spectrum 
analysis  is,  as  you  know,  generally  recognized  as  of  extreme 
delicacy,  and  yet,  so  far  as  the  demonstration  of  the  presence 
of  the  noble  metals  is  concerned,  these  colloid  methods  may 
not  only  equal  spectro-analytic  ones,  but  in  certain  cases 
even  exceed  them  in  sensitiveness.  The  smallest  amount 
of  gold  that  may  be  recognized  spectroscopically  is  r^OTo^ 
of  a  milligram,  y-g-of TO"O  °f  a  milligram  may  be  recognized 
by  colloid-chemical  means.  TffHihnF  °f  a  milligram  of 
platinum  may  be  recognized  spectroscopically,  while  Tmnfinnr 
suffice  for  its  recognition  colloid-chemically.  Spectrum 


SCIENTIFIC  APPLICATIONS  131 

analysis  is  more  sensitive  than  the  colloid-chemical  in  the 
case  of  silver,  yTruVW  °f  a  milligram  of  silver  being  recog- 
nizable by  the  first  method  against  y^JiHnnT  by  tne  second.1 

In  the  experiments  on  the  production  of  colloid  gold,  I 
emphasized  the  very  different  substances  that  may  be  used 
as  reducing  agents.  Certain  of  the  organic  reducing  sub- 
stances act  peculiarly  energetically  in  this  regard.  It  is  for 
this  reason  that  the  formation  of  colloid  gold  may  be  used 
for  demonstrating  the  presence  of  organic  reducing  materials. 
The  so-called  humic  acids,  for  example,  which  give  our  soils 
their  black  color,  and  which  are  usually  found  in  the  soil  in 
colloid  form,  reduce  gold  solutions  to  colloid  gold  in  such 
low  concentrations  that  this  reaction  has  long  been  used  to 
prove  their  presence.2 

The  principle  is  used  in  similar  fashion  in  LEY'S  test  for 
the  distinction  of  natural  from  artificial  honey.  In  this,  an 
ammoniacal  silver  nitrate  solution  is  reduced  by  the  addition 
of  a  few  drops  of  very  dilute  honey.  The  metallic  silver 
that  is  produced  assumes  the  reddish-yellow  color  of  the 
colloid  metal,  when  natural  honey  is  used,  while  a  darker, 
more  greenish,  precipitate  is  formed  by  artificial  honey. 
Traces  of  albumin  or  of  ethereal  oils  present  in  the  natural 
product  are  probably  responsible  for  the  difference.  Their 
" protective  action"  serves  to  maintain  the  colloid  silver  in  a 
higher  degree  of  dispersion  in  the  natural  product  than  hi 
the  artificial  one.  While  I  would  not  recommend  such  an 
attempt,  it  should  not  be  a  difficult  thing  for  a  colloid 
chemist  to  do  away  with  this  difference  between  the  naturally 
and  the  artificially  produced  honey  by  discovering  a  material 
which  when  added  to  the  latter  would  take  the  place  of  the 
natural  protective  colloid.  The  practical  result  of  such  an 
investigation  would  probably  then  bring  about  a  reversal 
in  the  application  of  LEY'S  test,  for  the  manufacturers 
would  in  this  case,  as  usually,  add  too  much  of  the 
substance. 

'  See  J.  DONAU,  Koll.-Zeitschr.,  2,  273  (1908). 
2  See  P.  EHRENBERG,  Koll.-Zeitschr.,  5,  30  (1909). 


132  COLLOID  CHEMISTRY 

§3. 

Let  me  touch  next  upon  some  applications  of  colloid 
chemistry  to  inorganic  and  photo-chemistry.  A  much  dis- 
cussed and  most  fascinating  problem  in  this  field  concerns 
the  chemical  nature  of  the  substances  comprising  the 
latent  picture.  It  is  a  well-known  fact  that  this  depends 
upon  the  presence  of  certain  reduction  products  of  silver 
haloids;  hi  other  words,  upon  the  presence  of  compounds 
which  contain  more  silver  and  less  halogen  than  is  repre- 
sented, for  instance,  by  the  formula  AgCl.  These  reduction 
products  of  " photo-haloids"  are,  moreover,  differently 
colored  (yellow,  red,  violet,  blue,  etc.).  Until  recently  it 
was  thought  that  they  all  consisted  of  " sub-haloids,"  com- 
pounds having  some  such  formula  as  Ag^Cl.  The  draw- 
backs to  this  view  reside  in  the  fact  that  such  sub-haloids 
have  never  been  isolated;  that  one  is  compelled  to  hold  to 
the  existence  of  a  whole  series  of  them  (as  a,  0,  7,  5,  etc., 
haloids)  having  different  colors;  and  that  they  must  be 
assumed  to  be  able  to  pass  easily  from  one  form  to  another. 
You  will  perhaps  at  this  point  recall  the  different  colors  that 
colloid  silver  assumes  and  so  yourselves  reach  a  conclusion 
which  has  been  well  developed  by  LUPPO-CRAMER  l  in  his 
numerous  and  careful  studies  of  the  subject.  The  photo- 
haloids  are  not  chemical  compounds  containing  silver  and 
halogen  in  stoichiometrical  proportions,  but  represent 
adsorption  complexes  of  colloid  silver  in  different  degrees  of 
dispersion  with  normal,  non-reduced  silver  haloids. 

The  correctness  of  this  view  has  been  demonstrated  by 
the  experiments  of  W.  REiNDERS,2  who  has  succeeded  in 
producing  these  photo-haloids  synthetically  in  the  form  of 
differently  colored  crystals  by  allowing  different  silver 
haloids  to  crystallize  in  the  presence  of  differently  colored 

1  See  the  numerous  papers  of  LUPPO-CRAMER  dealing  with  the  relations 
of  colloid  chemistry  to  photography  throughout  the  volumes  of  the  Kolloid- 
Zeitschrift.     See  also  his  books,  Kolloidchemie  und  Photographic,  Dresden, 
1908,  and  Kolloides  Silber  und  die  Photohaloide,  Dresden,  1908. 

2  See  W.  REINDERS,  Roll.  Zeitschr.,  9,  10  (1911)  where  references  to  the 
literature  may  be  found. 


SCIENTIFIC  APPLICATIONS  133 

colloid  silvers.  The  crystals  took  up  the  colloid  silver  and 
ultimately  appeared  with  its  color.  This  is  certainly 
beautiful  proof. 

In  passing,  I  should  like  to  point  out  that  numerous  other 
colloids,  as  the  organic  dyes,  may  be  thus  taken  up  by 
crystals.  Gelatin  may  also  be  absorbed.  These  facts 
deserve  much  consideration,  for  they  show  that  the  process 
of  crystallization  in  the  presence  of  a  colloid  does  not  always 
represent  a  purification  of  the  crystallizing  material.  Other 
illustrations  might  be  introduced  to  show  how  crystals  may 
take  up  colloids  as  impurities.  In  fact,  such  complexes 
have  led  to  erroneous  conclusions  regarding  the  existence  of 
different  chemical  compounds,  as  in  the  case  of  the  so-called 
chromisomers,  when  really  none  such  existed.1 

Time  permits  me  only  to  mention  the  fact  that  many 
allegedly  chemical  compounds  have  proved  to  be  colloid  in 
nature.  Many  of  the  hydrates,  for  example,  are  now  known 
to  be  colloid  or  adsorption  compounds,  as  illustrated  in  the 
different  silicic  acids  which  hold  water.2  On  the  other 
hand,  compounds  like  the  allegedly  different  stannic  acids 
have  turned  out  to  be  one  and  the  same  substance  existent 
in  different  degrees  of  dispersion,3  and  the  so-called  "  solu- 
tions" of  the  alkali  metals  and  of  silver  in  liquid  ammonia 
are  probably  of  colloid  nature.4 

§4. 

The  applications  of  colloid  chemistry  to  organic  chemistry 
are  not  only  already  very  extensive,  but  promise  to  multiply. 

1  Such  a  fictitious  color  isomerism  was  discovered,  for  example,  by  O. 
HAUSER,  Ber.  d.  Dtsch.  Chem.  Ges.,  45,  3516  (1912),  in  the  case  of  potas- 
sium ferrocyanid,  in  which  through  the  presence  of  colloid  berlin  blue  a 
fictitious  color  isomerism  was  brought  about.     I  believe  that  many  other 
alleged  examples  of  color  isomerism  depend  upon  just  such  colloid-chemical 
phenomena. 

2  See  the  numerous  papers  of  J.  M.  VAN  BEMMELEN  in  Die  Absorption, 
edited  by  WOLFGANG  OSTWALD,  Dresden,  1910. 

3  See   W.   MECKLENBURG,    Zeitschr.   f.   anorganische   Chemie.,   64,    368 
(1909);   74,  207  (1912);   a  review  is  found  in  Koll.-Zeitschr.,  11,  202  (1912). 

4  See  WOLFGANG  OSTWALD,  Kolloidchem.  Beihefte,  2,  437  (1908). 


134  COLLOID  CHEMISTRY 

All  those  sticky,  mucilaginous,  resinous,  tarry  masses  which 
refuse  to  crystallize,  and  which  are  the  abomination  of  the 
normal  organic  chemist;  those  substances  which  he  care- 
fully sets  toward  the  back  of  his  cupboard  and  marks  "not 
fit  for  further  use,"  just  these  are  the  substances  which  are 
the  delight  of  the  colloid  chemist.  For  hi  most  instances 
these  properties  are  the  properties  of  colloids,  more  par- 
ticularly of  that  group  of  them  known  as  the  solvated 
emulsoids. 

Among  the  solvated  colloids  appears  a  class  which  we  have 
not  discussed  as  yet,  namely,  that  of  the  isocolloids.1  Please 
recall  that  by  the  term  colloid  we  mean  nothing  more  than 
a  dispersoid  in  which  the  degree  of  dispersion  has  a  definite 
value.  Now  we  cannot  only  conceive  of,  but  we  actually 
know  of,  instances  in  which  the  dispersed  phase  and  the 
dispersion  medium  have  the  same  chemical  composition  and 
yet  the  two  do  not  form  a  homogeneous  or  molecularly 
dispersed  system.  Thus,  a  polymeric  compound  is  very 
frequently  not  molecularly  soluble  in  its  monomeric  form, 
and  this  is  true  for  many  pairs  of  chemical  isomers.  To 
illustrate  the  fact  in  entirely  modern  fashion,  we  need  but 
consider  the  artificial  synthesis  of  rubber  through  polymeriza- 
tion of  isopren.  Through  prolonged  heating,  the  isopren  is 
polymerized  to  a  colloid  product,  which  at  first  dissolves 
colloidally  in  the  monomeric  isopren,  greatly  increasing  its 
viscosity. 

Isocolloids  may  be  produced  from  a  single  chemical 
element,  for,  as  you  know,  certain  elements  may  exist  in 
different  so-called  allotropic  states.  These  may  then  be 
divided  colloidally  into  each  other.2  You  are  all  well  aware 

1  The  concept  of  the  isocolloid  was  first  set  up  and  developed  by  me  in 
my  Grundriss  der  Kolloidchemie,  2.  Aufl.,  128,  Dresden,  1911. 

2  The  objection  of  many  phase  rule  theorists  that  because  of  the  laws 
of  equilibrium  isodispersoids  cannot  exist,  looks  to  me  like  an  attempt  to 
do  violence  to  nature.     These  things  do  exist,  as  plainly  evidenced  by  the 
numerous  mixtures  which  consist  of  nothing  but  one  element  into  which 
that  same  element  has  been  dispersed  in  allotropic  form.     See  my  Grundriss 
der  Kolloidchemie,  2.  Aufl.,   128,  Dresden,   1911.     To  the  examples  given 
there  might  be  added  other  metals,  as  tin  and  zinc.     Just  because  these 


SCIENTIFIC  APPLICATIONS  135 

that  silver  or  phosphorus,  for  example,  may  appear  in  a 
whole  series  of  different  physical  states  or  allotropic  forms. 
In  keeping  herewith,  we  know  of  a  whole  series  of  mixtures 
of  such  allotropic  modifications  which  have  been  proved, 
or  may  be  proved,  to  be  colloid  mixtures.  White  phos- 
phorus, on  exposure  to  light,  gradually  changes  to  the  red 
form.  When  this  change  is  studied  ultramicroscopically, 
the  allotropic  transformation  may  be  observed  directly. 
Under  the  influence  of  the  light,  particles  of  colloid  dimen- 
sions are  produced,  which  gradually  coalesce  to  form  larger 
aggregates  with  a  net  or  honeycomb  structure.1  In  silver 
melts  at  temperatures  between  160  and  200°  C.,  we  probably 
also  deal  with  colloid  emulsoids  consisting  of  two  forms  of 
silver.  This  is  rendered  probable  by  the  fact  that  the 
viscosity  changes  observed  in  this  temperature  realm  are 
identical  with  those  observable  in  typical  colloid  mixtures.2 
To  these  isodispersoids,  more  particularly  the  isocolloids, 
belong  also  the  various  resins,  many  oils  and  probably  the 
majority  of  those  troublesome  organic  residues  which  fail  to 
crystallize. 

But  illuminating  results  will  also  follow  the  application  of 
colloid  chemistry  to  other  branches  of  organic  chemistry,  as 
to  that  of  the  dyestuffs.  They  are  certainly  to  be  expected, 
for  a  large  number  of  the  organic  dyestuffs  form  typical 
colloid  solutions  in  water.  Since  the  properties  of  such 
solutions  depend  upon  the  degree  of  dispersion,  it  is  to  be 
expected  that  changes  in  this  colloid  state  of  the  dyestuffs 
cannot  fail  to  be  of  importance  in  the  dyeing  properties  of 
the  dyes  themselves.  Colloid  chemistry  has  already  been 
asked  to  shed  light  upon  certain  dyestuff  problems,  where 
attempts  to  explain  the  phenomena  observed  through  the 

systems  do  not  fit  into  theories  of  equilibrium,  they  do  not  therefore  dis- 
appear from  nature,  nor  do  they  lose  in  this  fashion  their  great  scientific 
and  practical  significance. 

1  See  H.  SIEDENTOPF,  Ber.  d.  Dtsch.  Chem.  Ges. 

2  See  WOLFGANG  OSTWALD,  Handbook  of  Colloid  Chemistry,  Trans,  by 
FISCHER,  OESPER  and  BERMAN>  Philadelphia,  1915.     See  also  Koll.-Zeitschr., 
12,  220  (1913). 


136  COLLOID  CHEMISTRY 

orthodox  ones  of  chemical  constitution  have  given  only 
arbitrary  explanations  or  failed  entirely.  It  has  been  found 
that  a  large  number  of  the  colloid  organic  dyestuffs  behave 
as  do  the  colloid  metals.  They  change  their  color,  for 
instance,  with  the  degree  of  their  dispersion,  passing  from 
yellow  through  red  to  blue  just  as  do  colloid  gold  or  silver.1 
This  behavior  was  to  be  expected,  for  the  properties  of  the 
organic  dyestuffs  are  so  similar  to  those  of  the  metals  that 
the  larger  text-books  of  physics  discuss  the  optics  of  the 
metals  and  of  the  organic  dyestuffs  in  the  same  paragraphs. 

§5. 

Colloid  chemistry  finds  many  applications,  too,  in  the 
realm  of  its  sister  science,  physical  chemistry.  There  exists 
the  closest  possible  relation  between  colloid  chemistry  and 
capillary  chemistry.  Colloid  chemistry  is  in  reality  nothing 
but  a  special  division  of  capillary  chemistry,  for  both  deal 
chiefly  with  systems  which  consist  essentially  of  surface. 
On  the  one  hand,  the  phenomena  of  surface  tension,  of 
adsorption  and  of  capillary  electricity  find  immediate  appli- 
cation to  colloid  chemistry,  while  this  sheds  new  light  into 
the  field  of  capillary  electricity. 

But  there  also  exist  relationships  between  colloid  chemis- 
try and  more  distant  realms  of  physical  chemistry.  As  you 
know,  the  classical  solution  laws  of  VAN'T  HOFF  and  others 
begin  to  show  exceptions  when  concentrated  solutions  are 
studied.  Now  call  to  mind,  in  this  connection,  what  has 
previously  been  emphasized,  that  these  concentrated  solu- 
tions often  exhibit  the  earmarks  of  the  colloid  state  by 
showing  the  TYNDALL  phenomenon,  by  becoming  viscid, 
etc.  It  has  been  suggested  recently,  especially  through  the 
work  of  American  investigators,  that  in  these  concentrated 
molecular  solutions  there  occurs  a  fusing  of  the  dissolved 
particles  with  the  solvent  —  in  other  words,  solvation.  It 
is  assumed  that  the  ions  or  molecules  of  the  dissolved  sub- 
stance unite,  under  certain  circumstances,  with  a  large 

1  See  Kolloidchem.  Beihefte,  2,  409  (1911). 


SCIENTIFIC  APPLICATIONS  137 

number,  one  hundred  or  more,  of  the  molecules  of  the  dis- 
persion medium.  No  one  seems  thus  far  to  have  even 
hinted  that  whenever  a  thousand  molecules  unite  hi  this 
fashion,  complexes  of  colloid  dimensions  must  result  as  a 
matter  of  necessity.  In  such  solvates  the  amount  of  the 
dispersion  medium  bound  by  the  molecules  varies  progres- 
sively and  cannot  therefore  be  expressed  through  simple 
stoichiometrical  relations.  Careful  study  of  the  problem 
shows  that  between  the  laws  governing  such  solvation,  and 
those  which  govern  the  formation  of  adsorption  compounds1 
there  exists  a  whole  series  of  analogies.  As  previously 
noted,  solvation  is  characteristic  of  a  large  number  of  col- 
loids. A  particle  of  albumin  or  gelatin,  for  example,  readily 
holds  a  thousand  times  its  own  weight  of  water. 

These  considerations  must  render  it  apparent  that  we  are 
destined  to  discover  solution  laws  which  will  embrace  both 
these  classes  of  dispersoids.  The  coarse  colloids  (perhaps 
even  the  coarse  dispersions)  will  occupy  one  of  the  extremes 
under  these  laws,  the  dilute  molecular  dispersoids  the  other. 
At  the  present  moment  these  laws  are  still  undiscovered, 
but  I  believe  that  the  view  here  expressed  will  bear  better 
fruit  than  the  attempt,  constantly  made  now,  to  make 
adequate  the  laws  governing  dilute  solutions  by  ever- 
changing  additions  and  corrections.  Since  the  behavior  of 
the  molecular  dispersoids  passes  gradually  over  into  that  of 
the  colloid  systems,  such  more  general  laws  as  are  here 
discussed  would  naturally  embody  from  the  beginning  the 
corrections  which  need  constantly  to  be  made  in  laws  govern- 
ing dilute  solutions  when  the  attempt  is  made  to  make  these 
cover  the  anomalous  behavior  of  the  concentrated  molec- 
ular solutions. 

§6. 

It  is  not  a  mere  accident  that  the  three  most  modern 
branches  of  physical  chemistry  —  those  of  catalysis,  of  the 
crystalline  liquids  and  of  the  radio-active  substances  —  show 
1  See  Koll.-Zeitschr.,  9,  189  (1911). 


138  COLLOID  CHEMISTRY 

interesting  relationships  to  colloid  chemistry.  So  far  as 
catalysis  is  concerned  —  the  science  of  the  changes  in  the 
rate  of  a  reaction  through  the  presence  of  an  added  sub- 
stance which  does  not  appear  in  the  products  of  that  re- 
action —  I  pointed  out  in  the  second  lecture  that  colloids 
are  peculiarly  active  as  catalyzers.  But  not  only  do  colloids 
themselves  bring  about  such  catalytic  effects,  but  other 
materials  rich  hi  surface,  even  though  not  colloidally  dis- 
persed, act  in  similar  fashion.  I  need  but  point  out  the 
contact  effects  exhibited  by  platinum  black  and  other 
metallic  powders  in  the  production  of  sulphuric  acid;  and 
the  use  of  finely  powdered  metallic  hydroxids,  etc.,  in  various 
catalyses  as  developed  by  J.  SABATIER.  It  has  also  been 
discovered  that  the  effect  of  the  walls  of  the  containing 
vessels,  as  in  various  gas  reactions,  is  in  large  measure 
dependent  upon  their  roughness. 

If  we  try  to  say  how  the  element  of  surface  favors  such 
" heterogeneous  catalyses,"  the  effects  of  adsorption  at  once 
come  to  mind.  Adsorption,  as  previously  emphasized, 
depends  not  only  upon  the  absolute  but  upon  the  specific 
surface  (the  quotient  of  surface  divided  by  volume  or  weight). 
Therefore  when  the  absolute  or  relative  surface  in  a  reaction 
system  is  increased,  it  means  increased  adsorption,  and  such 
increased  adsorption  means  increase  in  the  concentration  of 
the  reacting  materials.  This  process  may  be  further  aided 
through  the  local  production  of  heat  which  so  frequently 
accompanies  adsorption.  The  two  processes  together  will 
serve  to  explain  a  large  part  of  the  increase  in  reaction  rate 
seen  in  these  systems.  Secondary  chemical  reactions  may 
also  be  made  responsible  for  a  group  of  these  catalytic 
effects.  Such  secondary  chemical  reactions  are  also  rendered 
possible  through  adsorption  effects.  As  previously  explained, 
the  great  or  specific  adsorption  of  one  of  the  constituents  of 
a  reaction  mixture  may  change  the  whole  system  of  chemical 
equilibrium  in  that  mixture.  These  facts  will  suffice  to 
show  why  it  is  a  matter  of  paramount  importance  that  the 
ferments  of  the  living  organism  are  for  the  most  part  colloid 


SCIENTIFIC  APPLICATIONS  139 

in  nature  and  why  a  study  of  their  reactions  from  the  point 
of  view  of  colloid  chemistry  and  of  adsorption  catalysis 
promises  so  very  much. 

A  second  subject  in  physical  chemistry  to  which  colloid 
chemistry  finds  application  concerns  the  liquid  crystals, 
or,  as  they  are  better  called,  the  crystalline  liquids,  those 
peculiar  substances  whose  refraction  behavior  relates  them 
closely  to  the  solid  crystals.  Those  of  you  who  are  in- 
terested in  these  substances  will  know  that  a  question 
concerning  them  has  long  been  discussed,  which,  in  start- 
ling fashion,  is  practically  identical  with  that  which  is 
constantly  raised  in  the  problems  of  colloid  chemistry. 
The  question  at  stake  is  whether  the  crystalline  liquids, 
which  at  times  are  distinctly  turbid  or  refractively  colored 
and  often  highly  viscid,  are  homogeneous  or  molecularly 
dispersed  systems,  or  whether  they  are  heterogeneous  sys- 
tems of  the  type  of  the  emulsions.  As  in  the  case  of  the 
colloids,  many  facts  seemed  to  argue  for  the  first  of  these 
conceptions  and  many  for  the  second.  But  just  as  with 
the  colloids,  discussion  of  this  problem  has,  strictly  speak- 
ing, brought  no  decision  either  way.  The  most  widely 
accepted  theory  of  the  classical  physical  chemists  is  that 
of  M.  BOSE,  which  holds  that  swarms  of  molecules,  in 
other  words,  loose  combinations  of  a  number  of  molecules, 
swim  about  in  the  crystalline  liquid.  But  this  concept 
of  a  " swarm  of  molecules"  is  evidently  nothing  more  than 
the  designation  of  submolecular  or  colloid  aggregates;  and 
even  though  this  word  " colloid"  has  entered  into  the 
discussion  only  recently,1  more  and  more  evidence  is  ac- 
cumulating to  indicate  that,  at  least  in  many  instances, 
the  crystalline  liquids  are  typical  emulsion  colloids. 

Besides  the  fact  that  these  liquids  are  turbid  or  opal- 
escent, we  need  but  emphasize  their  peculiar  behavior  re- 
garding changes  in  viscosity  when  they  are  chilled.  The 
viscosity  curve  again  assumes  a  form  identical  with  that 
observed  when  separation  phenomena  occur  in  critical 

1  See  Koll.-Zeitschr.,  8,  270  (1911). 


140  COLLOID  CHEMISTRY 

fluids,  as  during  the  coagulation  of  albumin  by  heat,  in 
the  separation  of  sulphur  melts,1  etc. 

That  these  crystalline  liquids  have  a  colloid  degree  of 
dispersion  has  also  been  proved  directly,  in  many  instances, 
by  means  of  the  ultramicroscope.  Further  evidence  hi 
this  direction  is  offered  by  their  great  sensitiveness  to 
chemically  indifferent  substances,  so  far  as  changes  in  their 
optical  properties  are  concerned,  as  well  as  by  the  fact 
that  D.  VORLANDER  observed  the  development  of  an 
anisotropic  liquid  from  simple  mixture  of  two  isotropic 
liquids  which  in  no  way  react  chemically  with  each  other. 
We  find,  in  short,  a  constantly  growing  number  of  analo- 
gies between  crystalline  liquids  and  colloidally  dispersed 
systems.  This  classification  of  the  crystalline  liquids  with 
the  colloid  dispersoids  does  not,  of  course,  explain  their 
optical  peculiarities,  but,  by  following  a  lead  which  con- 
cerns an  almost  forgotten  microscopic  phenomenon  of 
capillarity,  it  is  possible  that  light  may  be  found. 

Entirely  normal,  isotropic  liquids,  like  water,  show  dis- 
tinct polarization  phenomena  when  they  are  observed  in 
a  dispersed  state,  in  other  words,  in  droplet  form.2  The 
phenomenon  has  been  designated  surface  polarization  and 
has  been  attributed  to  the  action  of  surface  tension,  which, 
in  tiny  droplets,  comes  to  assume  a  considerable  value. 
The  amount  of  this  surface  polarization,  on  the  one  hand, 
increases  with  increasing  degree  of  dispersion.  On  the 
other  hand,  its  type,  sign  and  value  must  be  influenced  by 
the  chemical  nature  and  perhaps  the  shape  of  the  mole- 
cules. It  may  be  —  I  make  this  suggestion  with  all  re- 
serve—  that  we  find  here  a  bridge  between  the  surface 
tension  phenomena  of  microscopic  droplets  and  the  optical 
properties  of  systems,  which  like  the  colloids,  consist  almost 
entirely  of  surface. 

The  third  modern  —  perchance  most  modern  —  branch 

1  See  Koll.-Zeitschr.,  12,  213  (1913). 

2  See  V.  VON  EBNEE,  Untersuchungen  tiber  der  Anisotropie  organisierter 
Substanzen,  2,  Leipzig,  1882;  O.  BUTSCHLI,  Untersuchungen  iiber  Strukturen, 
31,  35,  Leipzig,  1898,  where  references  may  be  found  to  earlier  observations. 


SCIENTIFIC  APPLICATIONS  141 

of  physical  chemistry,  into  which  colloid  chemistry  has 
recently  penetrated,  is  that  of  radio-chemistry.  Some  years 
ago  I  suggested  that  it  would  be  an  especially  interest- 
ing feat  in  synthetic  colloid  chemistry  could  the  radio- 
active elements  be  obtained  in  colloid  form.1  Two  years 
ago  it  was  shown  that  nature  had  already  made  this 
experiment.  It  was  found  that  a  whole  series  of  aqueous 
solutions  of  radioactive  substances  are  colloid  in  nature.2 
These  solutions  show  the  phenomena  of  electrophoresis, 
of  coagulation  through  electrolytes,  they  do  not  diffuse 
or  dialyze,  are  easily  adsorbed  through  other  colloids,  etc. 
Not  all  radioactive  substances,  but  the  majority,  are 
found  in  this  colloid  state;  hence  colloid-chemical  methods, 
as  those  of  dialysis,  absorption,  etc.,  may  be  utilized  to 
accomplish  their  separation  and  concentration.  This  is 
certainly  a  very  startling  and  interesting  application  of 
colloid  chemistry. 

§7. 

But  observations  upon  dispersed  (more  particularly 
colloidally  dispersed)  systems  have  yielded  valuable  fruit 
in  another  field  which  interests  chemists  and  physicists 
alike.  I  refer  to  the  experimental  determination  of  Avo- 
GADRO'S  constant,  the  famous  value  N,  which  states  how 
many  molecules  are  contained  in  a  gram-molecule  of  any 
substance.  There  exist  different  methods  by  which  this 
fundamental  figure  may  be  determined,  of  which  I  shall 
mention  only  the  following. 

As  you  know  the  atmosphere  surrounding  the  earth  be- 
comes rarer  as  we  ascend.  The  matter  is  expressed  in  the 
law  that  with  arithmetic  progression  upwards,  the  density 
of  the  atmosphere  decreases  geometrically.  In  the  kinetic 
theory  of  gases,  this  law  may  be  used  for  determining  the 
value  of  AVOGADRO'S  constant  in  that  the  degree  of  baro- 

1  See  my  Grundriss  der  Kolloidchemie,  2.  Aufl.,  121,  Dresden,  1911. 

2  See  F.  PANETH,  Koll.-Zeitschr.,   13,    1,   297   (1913);    T.  GODLEWSKI, 
Koll.-Zeitschr.,  14,  229  (1914). 


142  COLLOID  CHEMISTRY 

metric  change  which  follows  the  unit  decrease  in  density 
is  inversely  proportional  to  the  gram-molecule  of  the  gas.1 
According  to  J.  PERRIN,  this  same  change  in  concentration 
under  the  influence  of  gravity  is  also  shown  by  dispersoids, 
provided  the  particles  are  so  small  that  they  show  BROWNIAN 
movement.  The  concentration  of  the  dispersed  substance 
in  any  mass  of  colloid  material  is,  therefore,  always  greater 
at  the  bottom  of  a  vessel  than  at  its  top.  At  a  given 
height  this  difference  is  the  greater,  the  coarser  the  dis- 
persion. In  suspensions  of  gutta-percha .  or  mastic,  in 
which  the  particles  are  about  0.3  /*  in  diameter,  the  con- 
centration of  the  particles  50  n  above  the  bottom  of  the 
dish  is  only  half  that  of  the  bottom,  while  in  the  case  of 
the  earth's  atmosphere,  the  density  does  not  fall  to  half 
that  obtaining  at  the  surface  of  the  earth  until  a  height  of 
six  kilometers  is  reached.  But  PERRIN  was  able  to  show 
that  the  same  law  not  only  holds  for  the  distribution  of 
gases  and  of  the  particles  in  coarser  dispersoids,  but  that 
one  can  calculate  the  value  of  AVOGADRO'S  constant,  when 
it  is  assumed  that  every  dispersed  particle  behaves  like  a 
molecule.  A  "  gram-molecule "  of  the  dispersed  particles 
would,  therefore,  equal  their  weight  times  N.  The  values 
thus  obtained  are  in  striking  accord  with  those  found  by 
other  methods  and  yield  a  value  of  6  to  7.1023  molecules 
in  the  gram-molecule. 

I  can  only  touch  upon  the  fact  that  AVOGADRO'S  constant 
can  also  be  calculated  from  the  velocity  of  Brownian  move- 
ment and  that  when  this  is  done  the  same  value  is  obtained. 
It  is  certainly  remarkable  that  from  the  observation  of  a 
single  particle  of  oil  or  mercury  —  even  from  the  study 
of  a  drop  of  diluted  milk  (DECKHUYZEN)  —  so  funda- 
mental a  value  as  that  of  AVOGADRO'S  constant  may  be 
calculated. 

1  For  details  see  J.  PERRIN,  Die  Atome,  Deutsch  von  A.  LOTTERMOSER, 
Dresden,  1914. 


SCIENTIFIC  APPLICATIONS  143 


I  beg  you  now  to  follow  me  to  still  greater  heights.  You 
may,  perhaps,  think  that  I  am  joking  when  I  say  that 
colloid  chemistry  has  already  found  interesting  applica- 
tions in  the  realm  of  cosmic  physics  and  that  in  the  future 
it  will  find  still  greater  ones.  Consideration  of  our  uni- 
verse will  at  once  reveal  to  you  that  in  it  we  deal  not  only 
with  bodies  of  great  mass  and  with  those  of  molecular 
dimensions,  but  that  it  also  betrays  the  presence  of  dis- 
persoids  possessed  of  very  different  degrees  of  subdivision. 

Of  special  interest  are  the  dispersoids  of  the  sky  as  ob- 
served in  atmospheric  dust  and  in  the  atmospheric  water 
(steam,  clouds,  fog,  rain  and  snow).  I  have  already  em- 
phasized that  the  blue  and  yellowish-red  colors  of  the 
heavens  depend  upon  the  dispersoid  nature  of  the  atmos- 
phere, and  that  these  color  effects  rest  upon  the  same 
grounds  as  the  opalescence  of  typical  colloids.  In  both 
instances  we  deal  with  a  selective  diffraction  by  particles 
of  a  diameter  less  than  the  length  of  a  light  wave.  The 
analogy  between  the  opalescence  of  the  heavens  and  that 
of  a  mastic  solution  is  so  great  that  the  same  formula 
governs  both  phenomena.  Even  the  polarization  effects 
observable  in  them  are  entirely  similar  in  the  two  cases.1 

I  should  like,  in  passing,  to  direct  attention  to  yet  an- 
other optical  effect,  in  which  the  presence  of  tiny,  light 
diffracting  particles  plays  a  great  role,  namely,  that  of 
ordinary  daylight.  If  the  light  of  the  sun  were  not  diffused 
through  the  dispersoids  of  the  earth's  atmosphere,  there 
would  be  no  daylight  in  the  ordinary  sense  of  the  term. 
The  sun,  like  the  moon,  would  stand  in  the  heavens  as  a 
bright,  burning  disc  upon  an  entirely  black  background. 
Wherever  the  sunlight  did  not  strike  directly,  there  would 
exist  deep  shadow;  there  would  exist  everywhere  a  garish 
contrast  between  the  lighted  and  the  unlighted.  In  short, 
the  world  would  look  entirely  different.  We  are  indebted 

1  See  J.  M.  PERNTER,  Denkschr.  Ak.  d.  Wiss.  Wien,  73,  301  (1901);  as 
well  as  his  Physikalische  Meteorologie. 


144  COLLOID  CHEMISTRY 

to  the  atmospheric  dispersoids  for  our  ordinary  "day- 
light." 

But  clouds  (which  are,  for  the  most  part,  dispersoids 
of  the  composition  gas  +  liquid)  also  behave  like  colloid 
systems.  It  is  difficult  to  determine  accurately  the  size 
of  the  water  droplets  composing  clouds,  but  we  seem  to 
deal  with  particles  of  approximately  colloid  dimensions. 
This  is  proved  not  only  by  the  fact  that  they  float  hi 
the  air,  but  also  by  the  degree  of  polarization  shown  by 
the  light  emanating  from  them.1  These  heavenly  disper- 
soids also  show  a  "coagulation"  which  is  typical  of  emul- 
soid  systems.  The  product  of  this  coagulation  we  call 
rain.  All  this  is  also  not  a  joke.  We  even  know  which 
factors  are  chiefly  concerned  in  producing  the  coagulation 
of  these  heavenly  dispersoids.  Electrical  changes  are  most 
important  in  that  these  bring  about  a  coalescence  of  the 
highly  dispersed  water  particles  and  so  lead  to  their  "pre- 
cipitation" in  the  true  sense  of  this  term. 

But  the  applications  of  colloid  chemistry  may  mount  still 
higher.  I  suspect  that  a  large  number  of  you  have  read 
SVANTE  ARRHENIUS'  interesting  volume  "Das  Werden  der 
Welten."  If  you  have,  you  will  recall  that  in  the  theories 
of  this  investigator  regarding  the  origin  of  the  earth,  two 
factors  play  an  especially  great  role,  namely,  light  pressure 
and  the  presence  of  the  universally  distributed  cosmic 
dust. lr  In  the  movement  of  cosmic  dust  through  light 
pressureJARRHENius  sees  one  of  the  most  important  reasons 
for  different  cosmic  phenomena.  In  the  production  of  a 
new  heavenly  body  a  shifting  and  an  accumulation  of 
cosmic  dust  brought  about  through  light  pressure  is  as- 
sumed to  play  an  important  role.  It  is  therefore  of  great 
interest  that  K.  SCHWARZSCHILD  and  certain  other  physi- 
cists have  calculated  that  the  size  of  these  cosmic  particles 
is  not  without  influence  upon  their  velocity.  Particles  too 
small  or  too  large  are  moved  less  easily  than  those  of 
medium  size  and  calculation  yields  an  optimum  for  move- 

1  See  the  literature  cited  in  the  footnote  on  page  143. 


SCIENTIFIC  APPLICATIONS  145 

ment  when  they  have  a  diameter  of  about  0.16  /z-  But,  as 
you  will  recall,  this  value  places  them  in  the  realm  of  the 
colloids.  The  size  of  cosmic  dust  is  therefore  ideal  for 
these  cosmic  displacements.  The  illustration  again  shows 
the  relation  between  degree  of  dispersion  and  properties 
which  attain  a  maximum  in  the  colloid  realm.  That 
colloid  chemistry  should  in  this  fashion  be  of  importance 
in  the  production  of  new  worlds,  —  more  than  this  could 
hardly  be  asked  of  it. 

§9. 

We  shall  now  leave  these  ethereal  regions  and  glance 
in  the  opposite  direction.  Mineralogy,  geology,  soil  chem- 
istry, agricultural  chemistry  —  these  are  the  sciences  in 
which  colloid  chemistry  has  found  brilliant  application. 
In  fact,  it  has  long  been  at  home  in  these  fields. 

Under  the  heading  of  mineralogy  we  naturally  deal  chiefly 
with  solid  colloids.  In  order  to  give  you  at  once  a  par- 
ticularly pretty  example,  I  present  these  specimens  of  blue 
rock  salt.  It  was  long  debated  to  what  this  blue  color 
is  due,  for  chemical  analysis  revealed  no  constant  differ- 
ences between  the  ordinary  colorless  rock  salt  and  this 
blue  product.  Organic  impurities  were  often  imagined  to 
be  responsible,  while  other  authors  held  that,  as  in  the 
" silver  haloids,"  blue-colored  sub-haloids  of  sodium  were 
responsible.  But  recent  experiments  —  more  particularly 
ultramicroscopic  and  synthetic  studies  —  have  demon- 
strated that  we  have  to  deal  with  a  colloid  subdivision  of 
metallic  sodium  in  the  solid  NaCl.  Ultramicroscopic 
examination  shows  blue  rock  salt  to  contain  numerous 
intensely  colored  and  fairly  regularly  arranged  colloid  par- 
ticles which  do  not  appear  in  the  colorless  mineral. 

The  blue  rock  salt  can,  moreover,  be  produced  artifi- 
cially, according  to  the  experiments  of  H.  SIEDENTOPF,  in 
the  following  manner.  A  piece  of  colorless  rock  salt  and 
a  piece  of  metallic  sodium  are  together  sealed  in  a  glass 
tube;  the  tube  is  evacuated,  arid  its  contents  then  heated 


146  COLLOID  CHEMISTRY 

to  above  the  vaporization  point  of  sodium.  The  rock 
salt  now  assumes  a  yellowish  color  which  ultramicroscopic 
investigation  shows  to  be  due  to  a  molecularly  dispersed 
solution  of  the  sodium  metal  in  the  rock  salt.  The  yellow- 
colored  preparation  is  then  carefully  heated  a  second  time 
to  definite  temperatures,  recooled  and  perhaps  heated 
again.  In  this  way  there  is  obtained,  first,  a  reddish- 
violet,  and  then  a  blue  rock  salt.  This  treatment  of  the 
yellow  preparation  brings  about  a  condensation  of  the 
originally  molecularly  dispersed  metal  into  larger  particles 
which  finally  assume  colloid  dimensions.  We  deal,  in 
other  words,  with  a  typical  colloid-chemical  condensation 
method. 

Let  me  emphasize  hi  passing  that  similar  procedures  - 
the  production  of  a  highly  dispersed  solution  which  through 
its  cooling,  reheating  and  subsequent  recooling  is  made 
to  yield  a  dispersoid  of  colloid  dimensions  —  are  used  to 
produce  gold  ruby  glass,  ultramarine,  certain  forms  of  steel 
and  many  of  the  organic  sulphur  dyes.  I  must  also  men- 
tion that  the  blue  color  of  rock  salt  is  identical  with  the 
blue  obtained  when  sodium  is  dispersed  electrically  in 
organic  solvents,1  or  when  it  is  dispersed  through  the 
effects  of  different  rays  like  the  emanations  of  radium. 
The  appearance  of  these  blue  preparations  in  nature  is,  per- 
haps, due  to  just  such  emanations  originating  in  the  slightly 
radioactive  potassium  salts  which  accompany  rock  salt. 

It  is  probable  that  the  colors  of  many  other  minerals, 
as  those  of  Certain  precious  stones,  are  dependent  upon 
the  presence  of  colloidally  dispersed  materials,  like  the 
colloid  hydroxids.2 

We  are  acquainted  with  a  large  number  of  other  phe- 
nomena in  mineralogy  in  which  the  colloid  nature  of  the 
observed  changes  is  only  just  becoming  known.  It  has 
actually  turned  out  that  the  different  degrees  of  dispersion 

1  See  WOLFGANG  OSTWALD,  Kolloidchem.  Beihefte,  2,  438  (1911). 

2  See  especially  C.  DOELTER,  Das  Radium  und  die  Farben,  Dresden, 
1910. 


SCIENTIFIC  APPLICATIONS  147 

observed  in  different  minerals  may  be  utilized  in  classify- 
ing them.  It  was  first  pointed  out  by  the  lamented  Aus- 
trian mineralogist,  F.  CORNU,  that  under  the  old  heading 
of  the  "hyaline"  minerals  we  deal  with  "mineral  gels."  1 
The  existence  of  minerals  in  these  highly  dispersed  forms 
is  so  common  that  CORNU  was  led  to  the  formulation  of 
his  theorem  of  the  isochemites,  which  states  that  there  exists 
for  every  crystallized  mineral  a  highly  dispersed  and  there- 
fore colloid  double.  Thus  we  know  silicic  acid  not  only 
in  its  crystalline  form  as  quartz,  but  also  in  a  gel  state  as 
opal.  The  latter,  aside  from  its  water  content  and  the 
presence  of  certain  impurities,  is  identical  in  composition 
with  the  former.  For  the  hydrated  crystallized  iron  oxid 
or  brown  iron  ore  we  have  a  double  in  the  so-called  stilpno- 
siderite  or  yellow  ochre;  for  the  anhydrous  crystallized 
red  iron  ore,  a  parallel  in  red  ochre;  for  the  crystallized 
sulphids  of  the  heavy  metal  we  have  doubles  in  the  highly 
dispersed  "blacks"  and  "indigos"  (iron  black,  copper 
indigo,  etc.). 

Dispersoid  chemistry  can  also  teach  us  much  of  use  in 
the  classification  of  the  different  minerals.  I  show  you 
here  a  series  of  minerals  all  of  which  are  composed  of  silicic 
acid  alone  or  of  this  plus  water  (demonstration).  To  start 
with,  you  observe  the  well-known,  large  quartz  crystals, 
following  which  come  progressively  smaller  ones.  Next 
stands  the  so-called  chalcedony  which  no  longer  appears 
crystalline  even  under  the  microscope.  Then  comes  cacho- 
long,  for  which  the  same  is  true.  Here  I  show  you  the 
completely  amorphous,  glass-like,  hyalith,  which  already 
contains  several  percent  of  water.  Next  comes  the  so- 
called  siliceous  sinter  (fiorite,  geyserite)  and  then  the  opal 
which  in  its  "soft"  form  contains  thirty  to  forty  percent 
of  water.  As  the  last  member  in  the  series,  I  show  you  a 
normal  silicic  acid  gel  as  prepared  here  in  the  laboratory. 

1  Nearly  all  the  numerous  papers  of  F.  CORNU  and  his  collaborators  on 
the  relation  of  colloid  chemistry  to  mineralogy  may  be  found  in  the  Kolloid- 
Zeitschrift  from  the  fourth  volume  (1909)  on. 


148  COLLOID  CHEMISTRY 

I  have,  as  you  observe,  placed  a  series  of  silicic  acid 
minerals  before  you  possessed  of  widely  differing  degrees 
of  dispersion,  beginning  as  they  do  with  macroscopic  crys- 
tals and  terminating  with  a  typical  colloid.  But  this  is 
just  such  a  series  of  dispersoids  as  I  have  previously  shown 
you  in  the  case  of  sulphur  and  of  sodium  chlorid.  But  what 
is  most  important,  —  the  properties  of  these  minerals  from 
the  quartz  to  the  silicic  acid  gel  change  progressively  as  the 
degrees  of  dispersion  change. 

We  have  long  been  familiar  with  the  fact  that  there  exist 
minerals  of  which  we  cannot  say  definitely  whether  they 
belong  in  such  a  group  as  that  of  the  crystalline  quartzes  or  to 
the  microcrystalline  chalcedonies.  Even  chemical  methods, 
as  solubility  in  potassium  hydrate,  do  not  serve  to  distinguish 
them  sharply  from  each  other.  The  colloid  chemist  is  able 
to  show  why  these  analytical  methods  must  fail  and  why 
these  transition  forms  which  at  first  sight  prove  so  annoying 
to  the  systematist  are  bound  to  appear.  Our  second  lecture 
showed  the  solubility  of  silicic  acid  in  alkalies  to  vary  with 
its  degree  of  dispersion,  and  progressively  with  this.  In 
the  mineral  series  which  I  have  just  shown  you,  we  would, 
therefore,  expect  the  solubility  in  alkalies  to  increase  steadily 
from  the  crystallized  quartz  to  the  opal  and  the  colloid  silicic 
acid.  The  experiments  of  W.  MICHAELIS  on  the  solubility 
of  quartz  in  calcium  hydroxid  support  this  conclusion. 
Under  otherwise  constant  conditions,  he  observed  the  solu- 
bility of  a  smoothly  polished  quartz  crystal  to  be  about 
TTTnr  percent;  that  of  a  ground  crystal,  TV<r°o-  percent;  that 
of  melted  quartz  glass,  T-Jf -Q-  percent.  Finely  divided  but 
still  microscopically  visible  particles  of  quartz  powder 
allowed  12.4  percent  to  go  into  solution;  while  from  a  highly 
dispersed  quartz  powder  (in  which  the  individual  particles 
were  less  than  1  n)  practically  any  amount  could  be  brought 
into  solution  and  made  to  go  over  into  chemical  combination.1 

Similar  generalizations  hold  for  the  water  content  of  the 
quartzes.  This  also  increases  progressively  from  chalced- 

i  W.  MICHAELIS,  Koll.-Zeitschr.,  5,  9  (1909). 


SCIENTIFIC  APPLICATIONS  149 

ony  to  opal.  Colloid-chemically  it  follows  as  a  matter  of 
course  that  the  water  content  must  increase  with  every 
increase  in  degree  of  dispersion;  on  the  other  hand,  it  is  to 
be  expected  that  the  absolute  amount  held  may  vary  greatly, 
for  every  change  in  the  state  of  the  colloid  (as  induced,  for 
example,  through  admixture  with  impurities)  must  influence 
secondarily  the  water  absorption. 

Through  dispersoid  chemistry  we  may,  therefore,  gather  to- 
gether certain  mineral  groups  into  "  dispersoid  families,"  the 
individual  members  of  which,  so  far  as  their  physico-chemical 
properties  are  concerned,  pass  gradually  into  each  other. 

§10. 

Let  me  add  another  interesting  example  of  the  application 
of  colloid  chemistry  to  a  mineralogical  problem.  Yesterday 
I  showed  you  a  periodic  formation  of  precipitates  in  colloids, 
the  so-called  LIESEGANG  rings.  Many  of  you  no  doubt  at 
the  time  recognized  their  similarity  to  the  well-known  bands 
and  stripes  which  we  see  in  such  beautiful  form  in  agates, 
banded  jaspers,  etc.  Such  bands  are  also  seen  at  times  in 
certain  ores,  like  gold  ores.  This  similarity  between  the 
two  structures  is  more  than  a  merely  superficial  one.  Care- 
ful investigations  of  recent  date,  many  of  them  the  work  of 
R.  E.  LIESEGANG  himself,  have  shown  that  the  laboratory 
preparations  are  not  only  identical  in  appearance  with  the 
corresponding  minerals,  but  their  mode  of  production  is 
probably  the  same.  We  deal,  in  other  words,  in  these 
geologic  or  mineralogical  processes  with  the  diffusion  of 
molecularly  dissolved  substances  into  mineral  gels,  more 
particularly  into  silicic  acid  or  silicic  acid  gels  through  which 
a  periodic  precipitation  of  some  second  substance  dissolved 
in  the  gel  is  brought  about.  In  other  instances,  as  when 
the  agate  formation  occurs  about  a  central  nucleus,  the 
diffusion  and  the  periodic  precipitations  may  occur  centrif- 
ugally.  Details  regarding  the  whole  process  may  be  found 
in  R.  E.  LIESEGANG'S  volume.1 

1  R.  E.  LIESEGANG,  Geologische  Diffusionen,  Dresden  und  Leipzig,  1913. 


150  COLLOID  CHEMISTRY 

In  order  to  show  you  how  exactly  these  agate  formations 
may  be  imitated  in  the  laboratory,  I  have  prepared  the 
following  experiment  (demonstration).  You  will  recall  the 
periodic  precipitations  of  silver  chromate  which  are  obtained 
when  silver  nitrate  is  permitted  to  diffuse  into  a  gelatin  gel 
containing  potassium  bichromate.  If,  instead  of  allowing 
the  diffusion  to  occur  in  one  or  two  directions  only  (as  into  a 
test  tube  or  from  a  point  on  a  gelatin-covered  plate)  the 
diffusion  is  permitted  to  occur  in  three  directions,  what  I 
am  going  to  show  you  now  results.  A  rather  large  amount 
(say  500  cc.)  of  a  potassium  bichromate  gelatin  gel  is  pre- 
pared in  a  beaker  and,  after  the  whole  is  set,  the  solid  mass 
is  carefully  taken  out  of  the  beaker1  and  dropped  into  a 
somewhat  larger  one  containing  silver  nitrate.  Silver  nitrate 
surrounds  the  gelatinous  mass  on  all  sides,  and  therefore 
diffuses  concentrically  into  it.  After  about  twenty-four 
hours  the  silver  nitrate  solution  is  poured  off,  the  gelatin 
block  rinsed  in  water  and  placed  upon  a  dish  where  it  may 
be  sliced  open  with  a  large,  sharp  knife  (demonstration). 
If  the  experiment  has  gone  well  —  this  is  always  nervous 
work,  since  we  cannot  look  into  the  middle  of  the  gelatin 
during  the  experiment  —  the  gelatin  is  seen  to  be  streaked 
with  numerous  concentrically  arranged  bands,  which  yield 
different  pictures,  but  all  of  which  look  strikingly  like 
different  agates. 

I  should  like  to  add  that  we  observe  this  type  of  periodic 
structure  in  many  animals  and  plants  and  that  a  similar 
explanation  may  be  given  of  their  origin.  Let  me  direct 
your  attention  to  the  volume  of  E.  KUSTER,  which  deals  with 
the  biologic  applications  of  these  periodic  precipitations. 
Though  I  would  not  be  understood  as  maintaining  that  we 
can  at  once  explain  the  markings  of  a  zebra  or  a  tiger  in  the 
terms  of  colloid  chemistry,  still  there  is  no  doubt  that  impor- 

1  The  gel  is  best  removed  by  dipping  the  beaker  for  a  few  moments  into 
boiling  water.  If  the  instructions  given  on  page  109  are  followed,  it  is  best 
to  carry  out  the  experiment  in  a  refrigerator  in  order  to  obtain  a  thoroughly 
solid  gel.  It  is  necessary  to  use  a  good  quality  of  "hard"  gelatin  —  such  as 
is  used  in  bacteriology. 


SCIENTIFIC  APPLICATIONS  151 

tant   and  extensive  analogies  do  obtain  between  colloid 
chemistry  and  biological  phenomena. 

I". 

If  we  turn  to  geology,  the  important  effects  of  weathering 
immediately  give  rise  to  colloid-chemical  thoughts.  We 
are  here  again  indebted  to  F.  CORNU  for  pointing  out  that 
the  weathering  of  crystallized  minerals  nearly  always  yields 
gels  or  mixtures  of  gels.  From  feldspar  we  obtain  the  highly 
dispersed  kaolin;  from  serpentine,  talc;  from  brown  iron 
ore,  the  yellow  ochre,  to  which  the  yellow  color  of  clay  and 
earth  is  due. 

A  particularly  striking  example  of  the  by-effects  of  colloid- 
chemical  factors  is  seen  in  the  formation  of  deltas.  Delta 
formation  depends  upon  the  coagulation  of  grossly  dispersed 
and  colloid  materials  contained  in  the  sweet  waters  of  rivers 
by  the  electrolytes  of  sea  water.  Obviously,  this  coagula- 
tion will  occur  the  more  rapidly  and  be  the  more  intense  the 
more  concentrated  the  sea  water  which  meets  the  river 
water.  It  is  for  this  reason  that  the  unusually  high  salt 
content  of  the  Mediterranean  has  yielded  the  most  famous 
example  of  delta  formation,  namely,  that  of  the  Nile. 

§12. 

Soil  chemistry  has  also  to  do  with  many  different  disper- 
soids,  of  which  those  that  are  highly  dispersed  —  more 
particularly  colloidally  dispersed  —  are  especially  important. 
What  are  known  as  mechanical  methods  of  soil  analysis 
are  nothing  but  methods  of  dispersoid  analysis  —  coarsely 
dispersed  particles  are  separated  from  more  finely  divided 
ones  by  sieving,  by  sedimentation  and  by  filtration.  The 
colloidally  dispersed  phases  are  then  separated  from  each 
other  by  dialysis;  the  molecularly  dispersed,  by  processes 
of  diffusion. 

Of  the  typical  colloids  or  their  gels  which  we  find  hi  soils, 
four  kinds  deserve  particular  mention,  namely,  silicic  acid 
and  the  silicates,  aluminium  hydroxid  and  its  compounds 


152  COLLOID  CHEMISTRY 

with  silicic  acid  (in  other  words,  the  clays,  etc.),  iron  hy- 
droxid,  and  those  substances  rich  in  carbon  and  of  unknown 
chemical  composition,  summed  up  under  the  term  of  the 
humus  acids,  and  a  part  of  which  at  least  are  undoubtedly 
colloid.1  To  this  list  must  be  added  the  micro-organisms 
of  various  kinds  —  like  the  soil  bacteria  —  of  which  many 
are  so  small  that  suspensions  of  them  show  coagulation 
phenomena.2  We  must  also  add  the  mucinous  substances 
which  are  secreted  by  such  soil  organisms. 

For  determining  the  colloid  content  of  different  soils,  use 
has  recently  been  made  not  only  of  dialytic  procedures  but 
of  the  adsorption  of  dyes  like  malachite  green. 

The  important  role  of  the  colloids  in  the  soil  has  been 
more  and  more  emphasized  during  the  past  few  years  —  it 
has  in  fact  been  maintained  by  some  that  "the  fertility  of 
the  soil  is  proportional  to  its  colloid  content."  This  is 
certainly  carrying  it  too  far,  as  best  shown  by  the  fact  that 
a  whole  series  of  methods  for  improving  the  soil  consists  in 
producing  a  coagulation  of  the  soil  colloids.  The  good 
effects  of  frost  upon  a  soil  are  probably  due  to  such  a  coagu- 
lation. Laboratory  experiments  show  that  during  a  frost 

1  For  a  discussion  of  the  colloid  or  non-colloid  nature  of  the  humus  acids 
—  a  discussion  not  yet  ended  —  see  the  extensive  review  of  H.   BREHM, 
Koll.-Zeitschr.,  13,  19  (1913).     According  to  S.  ODEN  [Arkiv.  f.  Kemi.  usw., 
6,  Nr.  26  (1912);   Koll.-Zeitschr.,  14,  123  (1914)]  the  humus  acids  or  alkali 
humates  obtained  in  the  usual  fashion  from  peat  are  non-colloid  for  they 
dialyze,  show  no  ultramicroscopic  structure,  are  not  precipitated  by  salts, 
are  adsorbed  with  difficulty,  etc.     These  same  substances  separated  in  sim- 
ilar fashion  from  loam  by  Professor  SUZUKI,  working  in  my  laboratory,  be- 
haved in  typical  colloid  fashion.     They  dialyzed  but  little,  showed  a  distinct 
ultramicroscopic  structure,  were  easily  precipitated  by  sodium  chlorid,  were 
readily  adsorbed  by  bone  black,  etc.     These  facts  corroborate  the  general 
experience  of  chemists  that  the  humus  substances  may  appear  in  all  degrees 
of  dispersion  and  that  the  much  discussed  question  of  whether  they  are 
"colloid"  or  "molecular"  cannot  be  answered  by  yes  or  no.     This  question 
carries  a  different  answer  under  different  circumstances. 

2  According  to   E.   HILGARD,   A.   ATTERBERG,   etc.,   quartz   suspensions 
begin  to  show  coagulation  phenomena  when  their  particles  attain  a  size  of 
20  to  200  n.     In  this  connection,  and  for  a  general  discussion  of  the  relation 
between  colloid  chemistry  and  agricultural  chemistry,  see  P.  EHRENBEBG, 
Koll.-Zeitschr.,  3,  193  (1908);  4,  76  (1909);  6,  100  (1909). 


SCIENTIFIC  APPLICATIONS  153 

gels  are  formed  which  decrease  a  soil's  ''richness."  We  may 
explain  similarly  the  good  effects  of  " burning"  a  soil,  a 
practice  much  followed  formerly.  Under  this  heading  is 
also  to  be  put  the  application  to  the  soil  of  such  strongly 
coagulating  salts  as  calcium  sulphate.  All  these  methods 
not  only  bring  about  a  coarsening  of  the  soil  colloids,  but 
they  reduce  their  high  indices  of  swelling  which  constitute 
the  characteristic  element  of  excessively  "  rich  "  soils.  These 
facts  should  suffice  to  show  that  too  large  a  colloid  content 
does  not  represent  the  optimum  for  plant  growth. 

On  the  other  hand,  it  cannot  be  doubted  that  the  colloids 
are  not  only  important,  but  that  they  are  absolutely  essen- 
tial to  the  fertility  of  soil.  This  was  well  known  even  to 
the  old  agricultural  chemists  and  is  proved  directly  by  the 
knowledge  that  sandy  or  gravelly  soils  —  be  their  chemical 
composition  what  you  will  —  are  unfertile.  A  whole  series 
of  facts  serves  to  emphasize  the  importance  of  a  medium 
content  of  colloid  materials.  The  water  content  of  soil 
must  obviously  be  largely  regulated  through  the  presence 
of  hydratable  colloids.  Sandy  or  other  coarsely  dispersed 
soils  do  not  hold  rain;  neither  do  they  draw  up  water  from 
the  depths  as  readily  as  do  soils  containing  more  colloid 
material.  The  soil  colloids,  by  holding  the  water  which 
falls  upon  them  and  by  bringing  it  up  from  the  depths, 
fulfill  one  of  the  most  important  conditions  necessary  for 
the  growth  of  plants  upon  the  surface  of  our  earth. 

But  the  adsorption  power  of  the  soil  colloids  for  dissolved 
substances  is  also  of  tremendous  importance.  Agricultural 
chemistry  recognizes  this  in  two  directions.  There  is,  first 
of  all,  the  adsorption  of  nutritive  substances  necessary  for 
the  growth  of  the  plants;  on  the  other  hand,  there  is  the 
adsorption  of  materials  which  are  poisonous  to  plants,  or 
which  are  the  product  of  their  metabolic  processes.  Water 
plants  do  better,  for  example,  when  any  highly  dispersed 
powder  or  colloid,  such  as  carbon,  iron  hydroxid  or  silicic 
acid,  is  added  to  the  water. 

The  adsorptive  activity  of  the  soil  colloids  so  far  as  nutri- 


154  COLLOID  CHEMISTRY 

tive  substances  is  concerned  may  be  either  useful  or  per- 
nicious, depending  upon  the  concentration  of  the  substances 
present  and  the  intensity  of  the  adsorption  —  the  latter 
increasing,  other  things  being  equal,  with  the  increase  in 
colloid  content.  This  adsorptive  activity  has  a  favorable 
action  when  the  nutritive  substances  concerned  are  present 
in  relatively  low  concentrations.  They  are  then  gathered 
together  by  the  soil  colloids  and  brought  to  the  plant  in 
greater  amounts.  On  the  other  hand,  it  may  be  followed 
by  evil  consequences,  as  when  the  concentration  of  the 
nutritive  substances  thus  brought  about  exceeds  an  optimum 
-  an  effect  particularly  likely  to  be  produced  in  the  case  of 
the  salts  —  or  when  the  adsorptive  force  is  so  great  that  the 
nutritive  substances  are  held  too  firmly  by  the  soil  colloids 
so  that  the  plant  roots  can  no  longer  take  them  over  in 
optimum  amounts,  or  at  an  optimum  rate.  The  good 
effects  of  using  calcium  salts  after  fertilizing  soil  with  phos- 
phates depends  hi  major  portion  upon  the  coagulating  effects 
of  the  former,  which  thereby  antagonize  the  adsorption  of 
the  phosphoric  acid  by  the  soil  colloids.  Unfavorable  ad- 
sorption effects  undoubtedly  come  to  pass  when  the  soil 
contains  too  much  colloid  material.  These  equilibrium 
considerations  lead  to  the  same  conclusion  which  the  prac- 
tical workers  in  agriculture  have  so  long  held.  Under 
otherwise  constant  conditions,  a  medium  colloid  content  gives 
greatest  fertility.  But  this  medium  colloid  content  may  have 
different  absolute  values,  depending  upon  the  concentrations 
of  the  nutritive  materials  present  —  in  other  words,  depending 
upon  the  chemical  composition  of  the  soil  and  the  individual 
needs  of  different  plants.  It  seems  to  me  that  this  view  best 
coordinates  the  numerous  and  apparently  contradictory 
findings  of  different  students  of  the  question  covering  the 
relationship  between  soil  colloids  and  its  fertility. 

In  passing  I  should  like  to  mention  the  interesting  chemi- 
cal consequences,  like  the  so-called  adsorption  decompositions, 
which  often  follow  adsorption  in  soils.  Many  years  ago, 
J.  M.  VAN  BEMMELEN  showed  that  gels  absorb  the  potassium 


SCIENTIFIC  APPLICATIONS  155 

from  potassium  sulphate  solutions  and  not  the  sulphate  - 
this  being  followed  by  the  appearance  of  free  sulphuric  acid 
in  the  solution  undergoing  adsorption.  These  specific  ad- 
sorption phenomena  which  in  their  turn  may  be  followed 
by  tremendous  secondary  chemical  changes  undoubtedly 
play  a  great  role  in  the  dynamics  of  the  soil. 

§13. 

I  have  already  used  up  the  major  portion  of  lecture  tune 
and  yet  am  only  now  coming  to  perhaps  the  greatest  and 
most  interesting  of  all  the  scientific  applications  of  colloid 
chemistry.  I  refer  to  those  made  in  biology  and  medicine.1 
Colloid  chemistry  is  the  promised  land  of  the  biological 
scientist,  and  it  is  almost  impossible  for  the  enthusiastic 
colloid  chemist  not  to  become  poetical  in  this  region. 

As  you  know,  the  elements  necessary  for  life  may  be 
gathered  together  under  the  chemical  headings  of  the 
proteins,  the  lipoids,  the  salts  and  water;  but  the  physical 
and  the  physico-chemical  conditions  necessary  for  life  can- 
not be  more  accurately  or  more  concisely  summed  up  than 
in  the  words  all  life  processes  take  place  in  a  colloid  system. 
The"  colloid  state  is  the  means  of  integrating  biological 
processes.  More  correctly  expressed,  only  those  structures 
are  considered  living  which  at  all  times  are  colloid  in  com- 
position. 

It  is  self-evident  that  because  of  the  close  association 
between  colloid  chemistry  and  biology  the  number  of  in- 

1  It  is  impossible  to  list  a  series  of  papers  which  will  cover  adequately 
the  many  relations  of  colloid  chemistry  to  biology  and  medicine.  For  a 
first  orientation  in  this  field,  see  H.  BECHHOLD,  Die  Kolloide  in  Biologic  und 
Medizin,  Dresden,  1912,  where  numerous  references  will  be  found.  The 
physical  peculiarities  of  living  matter  with  due  emphasis  upon  its  colloid 
nature  are  discussed  in  L.  RHUMBLER,  Das  Protoplasma  als  physikalisches 
System,  Wiesbaden,  1914.  Larger  volumes  dealing  with  this  general  subject 
are  those  of  R.  HOBER,  Physikalische  Chemie  der  Zelle  und  Gewebe,  Leipzig, 
1912;  N.  GAIDUKOW,  Dunkelfeldbeleuchtung  in  der  Biologic,  Jena,  1911; 
F.  BOTTAZZI,  Handbuch  der  vergleichenden  Physiologic,  Jena,  1913.  A  par- 
ticularly important  volume  is  that  of  MARTIN  H.  FISCHER,  (Edema  and 
Nephritis,  2nd  edition,  New  York,  1915,  in  which  not  only  medical  but  many 
non-medical  problems  of  biological  interest  are  discussed. 


156  COLLOID  CHEMISTRY 

dividual  colloid-chemical  laws  which  hold  in  biology  must 
be  enormously  great,  for  since  organisms  are  merely  special 
instances  of  colloid  systems,  there  can  exist  no  biological 
problems  in  which  colloid  chemistry  must  not  play  some 
part.  The  colloid-chemical  point  of  view  permeates  biology 
from  its  beginnings  in  causal  morphology  to  its  endings  in 
chemical  physiology.  Bacteriologists,  physicians,  students 
of  experimental  morphology,  plant  physiologists,  all  are 
interested  in  colloid  chemistry  and  its  development.  The 
biologists  find  colloid  chemistry  useful  to  their  ends  as  is  no 
other  science.  It  has  not,  however,  been  pressed  into  their 
hands  ready-made;  from  the  earliest  days  they  have  them- 
selves furthered  the  principles  of  pure  colloid  chemistry  and 
then  applied  them  to  their  specific  problems.  Nothing 
demonstrates  better  the  close  relations  between  the  two 
sciences  than  the  fact  that  a  large  number  of  colloid  chemists 
entered  their  fields  from  biology  or  through  biology.  I  need 
but  mention  A.  FICK,  C.  LUDWIG,  F.  HOFMEISTER,  Wo. 
PAULI  and  F.  BOTTAZZI.  And  even  the  newest  chapters  in 
colloid  chemistry  are  indebted  for  their  rapid  and  magnifi- 
cent growth  in  no  small  fashion  to  the  interest  and  enthu- 
siastic cooperation  of  the  biological  colloid  chemists. 

§14. 

I  choose  arbitrarily  when  from  the  wealth  of  the  biological 
applications  of  colloid  chemistry  I  select  a  few  examples. 

Let  us  begin  by  asking  how  it  is  proved  that  living  sub- 
stance is  itself  colloid.  Chemical  analysis  shows  that  the 
characteristic  building  blocks  of  organic  material  is  protein. 
But,  as  my  previous  lectures  showed,  it  is  this  very  substance 
which  is  most  typical  of  the  colloids,  belonging,  as  it  does  to 
the  class  of  the  hydrated  emulsoids.  The  fact  that  great 
numbers  of  experiments  in  colloid  chemistry  are  made  with 
fresh  egg  albumin,  with  blood  serum,  with  muscle  juice, 
indicates  that  the  proteins  exist  in  colloid  form  in  the  living 
organism  and  that  they  are  not  " produced"  by  chemical 
methods.  Moreover,  it  is  possible  to  coagulate  all  or  parts 


SCIENTIFIC  APPLICATIONS  157 

of  a  living  cell  (like  the  flagellse  of  bacteria),  and  the  means 
employed  to  this  end  and  the  results  obtained  are  identical 
with  the  coagulation  of  proteins  in  test  tubes.  Finally  - 
and  this  is  perhaps  the  simplest  direct  proof  of  its  colloid 
nature  —  living  matter  may  be  studied  under  the  ultra- 
microscope.  This  experiment  can,  however,  not  be  carried 
on  very  long,  for  the  intense  light  needed  soon  kills  the 
organism  with  the  exhibition  of  the  signs  of  coagulation. 
When  the  colorless  plasma  of  an  alga  cell 1  is  thus  studied,  it 
is  seen  to  consist  of  a  mixture  of  numerous  particles  of  differ- 
ent sizes,  of  which  a  large  number  are  typically  colloid.  It 
is  of  special  interest  that  many  of  these  particles  are  in  active 
motion.  The  particles  approach  each  other,  separate, 
coalesce  to  form  larger  particles,  disappear  entirely  —  indeed 
he  who  observes  this  ultramicroscopic  picture  of  a  living 
cell  for  the  first  time  will  perhaps  be  inclined  to  hold  that 
the  "true"  life  of  any  cell  is  not  to  be  seen  except  ultra- 
microscopically.  In  this  he  is  to  a  certain  extent  at  least 
in  error,  for  the  movement  of  the  colloid  particles  in  living 
matter  is  nothing  more  than  the  same  movement  as  observ- 
able in  any  sol  showing  BROWNIAN  movement.  The  ultra- 
microscope  shows  the  plasma  of  living  cells  to  be  a  mixture 
of  hydrosols  of  different  degrees  of  dispersion.  This  con- 
clusion, therefore,  bears  out  the  results  of  chemical  analysis. 
There  is,  moreover,  no  contradiction  between  this  finding 
and  the  fact  previously  emphasized  that  ultramicroscopic 
analysis  often  fails  to  establish  the  colloid  character  of  the 
biocolloids.  This  is  because  of  their  great  hydration. 

It  may  now  be  asked  whether  this  mixture  of  hydrosols 
exists  in  a  form  in  which  the  dispersed  particles,  like  the 
proteins  or  lipoids,  float  about  in  the  dispersion  medium,  or 
whether  through  secondary  rearrangement  these  colloid 
particles  may  go  to  form  microscopically  visible  structures. 
The  biologists  among  you  will  be  well  aware  that  a  union 
of  particles  into  a  network  or  a  honeycomb  is  often  seen 
in  living  protoplasm.  There  are  some  authors  —  as  O. 

1  See  N.  GAIDUKOW,  Dunkelfeldbeleuchtung  in  der  Biologic,  Jena,  1913. 


158  COLLOID  CHEMISTRY 

BUTSCHLI  — who  have  held  that  such  a  subcolloid  structure  is 
characteristic  of  all  living  matter.  Can  the  colloid-chemical 
laboratory  explain  how  such  microscopic  structures  come  to 
pass  in  living  protoplasm?  It  can.  N.  BEYERINCK  dis- 
covered the  interesting  phenomenon  that  when  two  liquid 
colloids  like  gelatin  and  starch  are  mixed  in  definite  con- 
centrations, the  resulting  mixture  of  hydrosols  does  not 
show  the  particles  of  the  two  to  be  dispersed  in  colloid  form 
uniformly  throughout  the  liquid  but  that  one  of  the  colloids 
divides  itself  in  droplet  form  into  the  second.  The  mixture 
in  consequence  assumes  a  net  or  honeycomblike  appearance. 
The  structure  thus  produced  is  obviously  analogous  to  that 
seen  in  living  matter  when  studied  microscopically  or  ultra- 
microscopically. 


§15. 

A  question  much  discussed  in  general  biology  and  which 
has  been  answered  through  colloid  chemistry  asks  regarding 
the  physical  state  of  living  matter.  Is  protoplasm  solid  or 
liquid?  Time  has  shown  that  the  form  in  which  this  ques- 
tion is  asked  is  here  again,  as  so  often  the  case,  an  entirely 
wrong  one.  Protoplasm  is  neither  solid  nor  liquid  when 
compared  with  typical  solids  or  liquids.  Its  physical 
peculiarities  are  those  of  a  hydrated  emulsoid  which  may 
show  all  degrees  of  fluidity  ranging  from  those  values  which, 
on  the  one  hand,  are  characteristic  of  a  normal  liquid  to  those 
which,  on  the  other,  are  characteristic  of  a  solid.  As  gelatin, 
depending  upon  its  temperature  and  its  concentration,  may 
show  all  states  from  a  liquid  to  a  solid,  just  so  may  proto- 
plasm. As  a  dilute  gelatin  gel  —  and  we  shall  see  shortly 
that  living  protoplasm  is  just  such  a  dilute  colloid  —  unites 
within  itself  the  properties  of  a  liquid  and  of  a  solid,  just  so 
does  living  matter  show  properties  which  at  one  time  make 
us  think  it  fluid  and  at  another  solid.  Protoplasm  shows, 
for  example,  capillary  phenomena,  protoplasmic  streamings, 
vacuole  formation,  throws  out  pseudopods,  and  its  separated 


SCIENTIFIC  APPLICATIONS  159 

portions  form  droplets.1  All  these  are  the  properties  of 
liquids.  Upon  the  other  hand,  protoplasm  shows  a  plas- 
ticity and  a  maintenance  of  form  which  is  seen  only  in  solids. 
An  ameba  deformed  through  pressure  tends  gradually  to 
resume  its  spherical  form;  and  slight  but  persistent  pressure 
will  give  the  embryonic  cells  of  frog  eggs  some  other  than  a 
spherical  shape,  and  this  will  persist  for  hours. 

It  would  seem  that  living  matter  is  constantly  oscillating 
between  the  extremes  of  a  solid  gel  and  a  liquid  sol.  Such 
oscillations  probably  underlie  ameboid  motion  and  are  the 
cause  for  the  appearance  and  disappearance  of  the  numerous 
structures  seen  at  different  times  in  the  life  cycle  of  the  cell. 
The  German  investigator  L.  RHUMBLER,  who  has  studied 
the  physical  properties  of  living  matter  in  masterly  fashion, 
came  to  the  conclusion  that  only  a  specially  dispersed 
structure  —  a  "spumoid"  structure,  as  he  calls  it  —  can 
account  for  this  remarkable  combination  of  physical  proper- 
ties. He  himself  emphasizes,  however,  that  this  structure 
must  be  of  the  same  kind  as  that  possessed  by  any  hydrated 
emulsoid.  In  reality  the  physics  of  the  two  is  the  same. 

§16. 

Closely  related  to  the  physical  properties  of  living  matter 
is  its  great  water  content.  It  is  not  generally  recognized  how 
very  great  is  this  proportion  of  water  in  living  matter. 
More  than  half  our  body  weight,  for  example,  is  water  and 
marine  algae  and  jelly  fish  hold  as  much  or  more  than 
ninety-six  percent.  It  is  certainly  remarkable  that  these 
organisms  not  only  hold  their  shape,  but  move,  swim,  eat 
and  make  love  —  and  this  with  ninety-six  percent  of  water 
in  their  affection.  These  things  are  made  possible  through 
colloid  chemistry,  for  only  colloid  systems  —  in  other  words, 
the  gels  —  can  hold  their  shape  when  thus  rich  in  water. 
These  remarks  explain  why  the  biological  question  of  how 
this  water-holding  power  is  brought  about  and  how  it  is 

1  See  L.  RHUMBLER,  Das  Protoplasma  als  physikalisches  System,  Wies- 
baden, 1914. 


160  COLLOID  CHEMISTRY 

regulated  under  different  circumstances  becomes  a  problem 
in  colloid  chemistry. 

No  doubt  you  know  that  in  the  heydey  of  the  classical 
physical  chemistry  of  the  solutions,  other  and  non-colloid, 
namely,  osmotic  forces,  were  called  upon  to  explain  the 
absorption  and  the  movement  of  water  in  organisms.  It 
was  assumed  that  the  cell  membranes  found  in  the  tissues 
of  the  higher  animals  acted  as  osmotic  membranes  in  that 
they  gave  passage  to  water  but  did  not  permit  dissolved 
substances  like  the  various  salts  to  pass  through  them.  In 
this  way  through  concentration  differences  an  osmotic  move- 
ment of  water  was  brought  about  into  the  cell  and  thus  its 
cell  turgor  was  maintained.  The  modern  developments  of 
cellular  physiology  have  shown  more  and  more  clearly  that 
this  role  of  the  osmotic  forces  has  been  greatly  overestimated. 
It  may  now  be  said  that  there  exist  but  exceptional  instances 
in  which  may  be  discovered  any  fairly  complete  analogy 
between  the  laws  of  osmotic  pressure  and  those  which  govern 
the  absorption  of  water  by  a  cell.  Even  though  I  would  not 
hold  to  the  extreme  view  that  osmotic  processes  play  no  role 
whatsoever  in  the  processes  of  water  absorption  by  living 
organisms,  unprejudiced  consideration  of  the  facts  compels 
the  conclusion  that  besides  these,  or  better  expressed,  far 
transcending  these  in  importance,  entirely  different  forces 
determine  the  water  content  of  an  organism.  Not  the 
osmotically  active,  molecularly  dispersed  constituents  of  a  cell 
(more  particularly  the  salts  therefore)  but  the  plasma  colloids 
are  primarily  responsible  for  the  water  content  of  the  living 
organism  and  for  the  changes  which  this  shows. 

Looked  at  now  this  conclusion,  which  in  broad  form  was 
first  drawn  and  discussed  by  MARTIN  H.  FiscHER,1  seems 
almost  self-evident.  For  not  only  do  we  know  from  labora- 
tory experiments  that  the  emulsoids  obtained  from  living 
organisms  are  able  to  hold  enormously  large  amounts  of 
water  just  as  can  the  living  organisms  themselves,  but  we 
know  also  that  this  capacity  for  holding  water,  as  measured, 

1  See  the  literature  cited  in  the  footnote  on  page  155. 


SCIENTIFIC  APPLICATIONS  161 

for  instance,  through  viscosity  changes,  can  be  tremendously 
modified  by  apparently  trivial  and  widely  different  types 
of  changes  in  surroundings.  The  influence  of  electrolytes 
(such  as  acids,  bases  and  salts)  is  so  great  that  variations 
in  their  concentration  within  biological  limits  results  in 
marked  variations  in  the  water  content.  I  should  like  to 
emphasize  this  by  citing  an  example.  As  I  have  already 
shown  you,  acids  in  moderate  concentration  increase  tre- 
mendously the  amount  of  water  absorbed  by  gelatin,  fibrin 
or  egg  albumin.  The  influence  of  the  hydrogen  ion  is  so 
enormously  great  that  the  presence  of  even  such  a  "weak" 
acid  as  carbonic  acid  brings  about  a  distinct  increase  in 
swelling.  As  Wo.  PAULI  and  R.  CHIARI  have  shown,  the 
amount  of  water  absorbed  by  a  gelatin  plate  which  is  kept 
in  freshly  distilled  water  is  much  less  than  that  of  one  kept 
in  a  distilled  water  exposed  to  air  containing  a  little  carbon 
dioxid.  This  increase  in  swelling  may  be  used  as  an  indica- 
tor for  the  presence  of  hydrogen  ions.  These  things  indicate, 
at  the  same  time,  how  very  easily,  through  variations  in  the 
chemistry  of  living  organisms,  changes  may  be  brought  about 
in  living  matter  which  will  alter  its  water  content.  We  shall 
return  to  this  question  shortly. 

§17. 

Besides  these  structural  and  physical  peculiarities  common 
to  living  matter  which  are  newly  illuminated  or  explained 
through  colloid  chemistry,  there  exist  many  close  relations 
between  colloid  chemistry  and  the  more  purely  chemical 
and  physico-chemical  reactions  which  are  characteristic  of 
living  matter.  It  has  often  been  asked,  for  example,  how 
it  is  possible  that  so  many  different  reactions  may  take  place 
side  by  side  in  a  cell  (a  structure  which  consists  so  essentially 
of  liquid)  without  all  running  together  and  yielding  chaos. 
The  experiments  with  colloid  mixtures  mentioned  above 
show  that  two  colloid  substances  —  even  when  divided  into 
the  same  dispersion  medium  —  need  not  mix  with  each 
other,  but  can  continue  to  exist  side  by  side  in  the  form  of 


162  COLLOID   CHEMISTRY 

microscopic  droplets.  The  great  variety  of  microscopic  and 
ultramicroscopic  structures  observable  in  living  matter 
leads  to  the  conclusion  that  the  different  chemical  con- 
stituents of  the  protoplasm  —  more  particularly  those 
existent  in  colloid  form  —  may  in  similar  fashion  exist  side 
by  side  without  mixing.  As  F.  HOFMEISTER*  has  indicated, 
we  may  imagine  each  of  the  individual  droplets  to  be  a  tiny, 
special  laboratory,  in  which,  undisturbed  by  the  surround- 
ings, some  one  or  certain  few  reactions  take  place.  Such  a 
localization  of  chemical  processes  within  the  mixture  would 
further  be  aided  by  the  fact  that  a  large  part  of  the  reacting 
substances  and  of  the  reaction  products  are  in  themselves 
colloid  and  therefore  do  not  on  their  own  accounts  tend  to 
diffuse  and  so  mix  themselves  with  neighboring  substances. 

§18. 

But  the  colloid  state  plays  a  great  role  in  the  chemistry  of 
living  matter  in  yet  another  direction.  We  have  repeatedly 
emphasized  that  adsorption  processes  must  play  a  great  role 
in  catalysis,  that,  in  fact,  many  of  its  features  must  be  held 
to  be  the  direct  consequences  of  concentration  increases 
brought  about  in  surfaces.  A  colloid  mixture  of  the  type 
of  protoplasm  must  therefore  offer  peculiarly  favorable  con- 
ditions for  the  play  and  for  the  acceleration  of  chemical 
reactions.  It  is  consequently  not  to  be  wondered  at  that 
those  substances  which  are  to  be  counted  among  the  most 
fundamental  of  the  constituents  of  living  matter,  namely, 
the  ferments,  are  known  for  the  most  part  only  in  colloid 
form.  Living  matter  seems  to  be  a  meeting  ground  for 
adsorption  effects  and  colloid  catalyses. 

Allow  me,  after  this  general  survey,  to  touch  upon  a  series 
of  special  biological  problems  in  which  the  colloid-chemical 
point  of  view  has  brought  much  light.  The  choice  must 
again  be  arbitrary  and  my  review  most  superficial.  For 
further  details  I  direct  you  to  the  special  articles  which  deal 
with  these  problems.1 

1  See  the  footnote  on  page  155,  as  well  as  numerous  articles  in  the  Kolloid- 
Zeitschrift  in  which  references  to  other  striking  papers  may  be  found. 


SCIENTIFIC  APPLICATIONS  163 

§19. 

Colloid  chemistry  brings  us  light  in  even  those  first  of  all 
biological  processes  which  are  concerned  with  the  formation 
of  new  organisms,  namely,  the  phenomena  incident  to 
fertilization  and  the  early  development  of  the  embryo. 
You  perhaps  know  that  the  stimulus  to  the  development  of 
a  sea-urchin  egg,  for  instance  —  it  matters  not  whether  this 
be  brought  about  by  sexual  fertilization  or  by  so-called 
artificial  means  —  is  characterized  morphologically  by  the 
formation  of  a  so-called  astrosphere.  Rays  of  concentrated 
plasma  appear  either  in  the  immediate  vicinity  of  the 
nucleus  of  the  fertilized  egg,  or,  it  may  be,  in  other  portions 
of  the  egg  plasma.  These  rays  then  act  subsequently  as 
centers  toward  which  the  products  of  the  divided  nucleus 
move.  I  cannot  go  into  details,  but  in  spite  of  the  specific 
variations  which  appear  in  different  animals,  what  I  have 
described  is  constant  in  all  fertilization  and  cell  division. 

Closer  study  of  the  problem  proves  that  this  formation  of 
the  astrosphere  represents  a  special  form  of  coagulation  of 
the  plasma  colloids,  and  microscopic  observation  suffices  to 
show  that  we  are  dealing  with  a  localized  collection  of  water- 
poor  and  coarsened  plasma.  We  deal,  in  other  words,  with 
the  conversion  of  a  sol  into  a  gel.  Micro-dissection  proves 
this  without  the  question  of  a  doubt,  for  the  astrospheres 
may  be  pulled,  as  more  solid  masses,  out  of  the  relatively 
fluid  egg  plasma  (G.  L.  KITE). 

The  conclusion  that  this  phase  of  fertilization  represents 
a  coagulation  process  may  be  proved  by  yet  other  means. 
Those  of  you  who  have  followed  the  well-known  studies  of 
JACQUES  LOEB  and  of  other  investigators  of  the  problem 
of  artificial  parthenogenesis  will  know  that  the  unfertilized 
eggs  of  sea-urchins  or  star  fish  may  be  made  to  develop  by 
many  different  means.  Not  only  does  treatment  with  acids, 
alkalies  or  specific  ions  lead  to  this  result,  but  the  water 
extraction  incident  to  the  effects  of  neutral  salts  is  also 
effective.  Temporary  exposure  to  high  and  low  tempera- 


164 


COLLOID   CHEMISTRY 


tures,  exposure  to  other  colloids  (as  the  serum  of  higher 
animals),  treatment  with  organic  liquids  like  benzol  or  toluol 
and  even  mechanical  treatment  (such  as  shaking,  rubbing 
or  brushing)  accomplishes  artificial  development  in  the 
unfertilized  eggs  of  many  organisms.  What  have  all  these 
methods  in  common?  If  you  will  recall  what  I  said  yester- 
day regarding  the  alterations  in  the  colloid  state  that  may 
be  brought  about  through  trivial  external  changes,  it  will 
be  clear  to  you  that  all  these  methods  for  inducing  artificial 
parthenogenesis  are  such  as  lead  to  changes  in  the  colloids  - 
more  especially  to  their  coagulation.  All  the  listed  methods, 
to  which  more  might  be  added,  leading  to  the  development  of  an 
egg,  serve  also  to  produce  coagulation  in  protein  sols.  Con- 
versely we  may  say  that  we  hardly  know  a  method  of  pro- 
ducing such  protein 
precipitation  which 
when  properly  used 
may  not  also  be  em- 
ployed to  start  artifi- 
cial development.1 

This  coagulation 
theory  of  fertilization 
receives  pretty  support 
through  the  possibility 
of  causing  in  colloids 
and  colloid  mixtures  a 
localized  and  oriented 
coagulation  which  in 
structure  is  strikingly 
like  the  astrospheres  observed  in  developing  cells.  Fig.  41 
is  a  picture  of  such  an  artificially  produced  astrosphere 
taken  from  the  work  of  0.  BUTSCHLI  and  made  at  a  time 
when  the  colloid-chemical  theory  of  fertilization  which  I  have 
sketched  to  you  had  not  yet  been  born.  The  biologists 
among  you  will  grant  its  striking  similarity  to  the  real  thing. 

1  MARTIN   H.    FISCHER  and   WOLFGANG   OSTWALD,    Pfliigers   Archiv.   f. 
Physiologie,  106,  229  (1905). 


FIG.  41. 


SCIENTIFIC  APPLICATIONS  165 

The  primary  process  which  leads  to  development  in  an 
egg  is  seen,  therefore,  to  be  a  colloid-chemical  one  and  of  the 
nature  of  the  transformation  of  a  sol  into  a  gel.  But  please 
do  not  misunderstand  me  in  the  matter.  I  would  not  have 
you  think  that  this  explains  everything  that  there  is  to  the 
process  of  fertilization.  Many  different  chemical  processes, 
as  those  of  increased  oxidation,  for  example,  accompany  the 
astrosphere  formation,  but  these  appear  only  after  the  colloid 
changes  which  I  have  described  have  started  the  process. 
That  which  starts  development  is  colloid-chemical. 

§20. 

Swelling  phenomena  also  play  a  large  part  in  the  various 
phenomena  characteristic  of  growth.  Chemical  analysis 
shows  frog  larvae,  for  instance,  to  owe  their  enormous 
changes  in  weight  up  to  the  time  of  their  metamorphosis  to 
land  animals  to  be  determined  almost  exclusively  by  mere 
changes  in  the  amount  of  water  absorbed.  As  shown  in  the 
curves  of  Fig.  42,  the  increase  in  amount  of  solid  substance 
is  so  slight  that  at  the  time  when  the  frog  takes  to  the  land, 
it  consists  of  ninety- three  percent  of  water.1 

These  same  facts  hold  for  the  growing  parts  of  plants. 
A  change  in  the  osmotically  active  constituents  of  a  growing 
part  great  enough  to  account  for  these  enormous  water 
absorptions  is  unknown.  We  know,  however,  that  the 
growing  parts  of  many  plants  are  acid.  But  as  previously 
pointed  out,  acids  enormously  increase  the  water  absorbing 
powers  of  various  colloids  even  when  present  in  only  minimal 
concentrations.  These  things  point  clearly  enough  to  the 
importance  of  swelling  phenomena  in  growth  processes. 

When  the  developing  frog  becomes  a  land  animal,  it  loses 
much  water/  But  this  may  also  be  explained  colloid- 
chemically,  for,  as  I  have  previously  emphasized,  a  gel  has 
a  different  and  lower  swelling  point  when  in  equilibrium 

1  See  A.  SCHAPER,  Arch.  f.  Entwicklungsmechanik,  14,  356  (1902);  also 
WOLFGANG  OSTWALD,  Uber  die  zeitlichen  Eigenschaften  der  Entwicklungs- 
vorgange,  49,  Leipzig,  1908. 


166 


COLLOID  CHEMISTRY 


with  water  vapor  than  when  in  equilibrium  with  fluid  water. 
Colloid  chemistry  is  also  interested  in  the  fact  that  desert 
plants  often  show  an  acid  reaction  —  a  circumstance  which 
would  permit  them  not  only  to  take  up  more  water,  but  also 


6000 


6000 


4000 


3000 


2000 


1000 


10      20        30       4Q       50      60       70       80      90  Days 
FIG.  42.  —  Increase  in  weight  during  growth  of  frog  larvae. 

to  hold  better  such  as  has  been  absorbed  against  the  forces 
leading  to  drying.  The  diurnal  changes  in  the  reaction  of 
plants  are  probably  also  followed  by  similar  variations  in 
water  absorption  capacity. 

§21. 

Another  problem  to  the  solution  of  which  colloid  chemis- 
try has  been  called  is  that  of  the  nature  of  the  muscular 
contraction.  I  cannot  go  into  details,  but  I  should  like  to 


SCIENTIFIC  APPLICATIONS  167 

point  out  that  the  electrical,  chemical,  mechanical,  optical 
and  other  changes  incident  to  the  muscle  contraction  can 
all  be  best  understood  in  the  terms  of  colloid  chemistry.1 
The  essence  of  the  muscular  contraction  seems  to  reside  in  a 
transport  of  water  from  certain  of  the  structural  elements 
making  up  the  muscle  to  certain  other  contractile  elements. 
Differently  expressed,  a  migration  of  water  occurs  from  one 
colloid  to  a  second  making  up  this  tissue.  This  migration 
is  brought  about  through  a  production  of  acids  (more 
especially  of  lactic  acid)  in  the  muscle.  In  this  we  see  again 
the  so-widely  distributed  and  so  exceedingly  active  influence 
of  the  hydrogen  ions  upon  water  absorption  by  a  biocolloid. 
That  the  phenomena  of  swelling  as  observed  outside  of  the 
body  may  occur  with  a  rate  and  to  an  extent  demanded  by 
a  colloid-chemical  theory  of  the  muscle  contraction  —  this 
I  showed  you  yesterday  in  discussing  the  swelling  of  gutta- 
percha  leaves  and  of  gelatin.  In  discussing  swelling  I  also 
emphasized  that  an  amount  of  mechanical  energy  is  liberated 
which  is  entirely  adequate  to  explain  the  mechanical  phe- 
nomena incident  to  the  contraction  of  a  muscle. 

§22. 

Of  the  many  other  problems  in  physiology  which  seem 
accessible  to  colloid-chemical  analysis,  I  can  only  touch 
upon  that  of  secretion.  The  physiologists  among  you  will 
know  that  what  physiology  seeks  is  an  understanding  of  the 
nature  of  the  " driving"  forces  which  bring  about  the  secre- 
tion of  water  by  a  cell  or  tissue  —  at  times  even  against  the 
existence  of  a  counter-pressure  like  hydrostatic  or  osmotic 
pressure.  Perhaps  more  than  in  any  other  chapter  of 
physiology  do  we  in  this  problem  of  secretion  still  speak  of 
" vital"  forces.  Even  after  utilizing  the  newest  concepts 
of  physical  chemistry  in  addition  to  the  older  ones  of  filtra- 
tion, diffusion,  etc.,  we  still  have  much  left  to  be  explained. 
Here  again,  colloid  chemistry  is  acquainted  with  forces 

1  See  MARTIN  H.  FISCHER  and  W.  H.  STRIETMANN,  Koll.-Zeitschr.,  10, 
65  (1912);  see  also  WOLFGANG  PAULI,  Kolloidchem.  Beihefte,  3,  361  (1912). 


168  COLLOID  CHEMISTRY 

which,  so  far  as  we  can  see,  are  fully  able  to  meet  the  require- 
ment that  the  forces  producing  secretion  must  be  essentially 
independent  of  hydrostatic  and  osmotic  pressure  differences, 
while  it  makes  clear  at  the  same  time  the  nature  of  a  number 
of  the  phenomena  which  commonly  accompany  such  secre- 
tion. I  refer  to  the  phenomena  of  secretion  observable  in 
simple  colloids,  and  discussed  in  the  last  lecture  under  the 
heading  of  syneresis. 

I  ask  you  to  recall  that  every  secretion  springs  from  a 
colloid  matrix,  and  that  the  secretion  contains  not  only 
water  but  colloids  and  salts  and  these  of  the  kind  present 
in  the  secreting  tissues  themselves.  Even  the  most  watery 
of  the  secretions,  like  the  urine,  contains  a  series  of  non- 
dialyzable  substances,  the  so-called  "  colloid  nitrogen. " 
What  is  true  of  secretion  is  also  true  of  syneresis.  The 
serum  squeezed  off  contains  not  only  water  but  also  colloids 
and  salts  and  these  in  proportions  which  need  in  no  sense  be 
identical  with  those  existent  in  the  secreting  gel.  In  synere- 
sis in  colloids,  as  in  physiological  secretion,  both  the  amount 
and  the  composition  of  the  serum  given  off  varies  not  only 
with  the  kind  of  colloid  but  with  the  kind  and  the  amount 
of  the  material  contained  in  it,  etc.1  But  what  is  most 
important  is  that  the  syncretic  secretion  of  fluid  is  not 
determined  through  osmotic  or  hydrostatic  pressure  differ- 
ences, but  is  dependent  upon  forces  existent  " within"  the 
gel  itself  —  upon  forces,  in  other  words,  which  lead  to 
changes  in  its  "  internal  state."  With  these  suggestions  I 
must  let  the  matter  rest. 

§23. 

Another  physiological  problem  much  studied  recently 
is  that  of  vital  staining,  in  other  words,  the  taking  up  of 
dyes  by  living  cells.  It  has  become  increasingly  evident 
that  the  degree  of  dispersion  of  the  dye  is  a  factor  of  prime 
importance  in  bringing  about  positive  results.2  As  a  rule 

1  See  pages  92  and  93. 

2  See  especially  W.  RUHLAND,  Koll.-Zeitschr.,   12,    113   (1912);    14,   48 
(1914),  where  the  literature  is  cited. 


SCIENTIFIC  APPLICATIONS  169 

only  molecularly  or  highly  dispersed  dyestuffs  can  be  taken 
up,  the  plasma  film  surrounding  cells  seeming  to  act  like 
an  ultrafilter. 

I  would  like  to  add  a  word  here  regarding  our  methods 
of  fixation  and  staining  of  dead  tissues.  Some  twenty 
years  ago  the  biologists  were  much  frightened  when  the 
botanist  A.  FISCHER  pointed  out  that  many  of  the  struc- 
tures found  after  such  treatment  are  "  artifacts."  Fixing 
and  staining  reagents  bring  about  dehydration  and  coagu- 
lation effects  —  in  other  words,  colloid-chemical  changes  - 
in  the  state  of  the  tissue  colloids.  It  is  undoubtedly  true 
that  many  things  may  be  seen  in  such  fixed  tissues  which 
it  would  be  wrong  to  say  exist  in  living  protoplasm;  on 
the  other  hand,  it  would  be  just  as  wrong  to  hold  that 
such  fixation  methods  can  tell  us  nothing  whatsoever  re- 
garding the  structure  of  living  matter.  A  rational  fixa- 
tion and  staining  technique  can  apprise  us  of  the  character 
of  the  changes  wrought  by  fixation  and  staining  reagents 
in  test  tube  experiments  upon  such  highly  hydrated  mix- 
tures as  are  presented  by  the  biocolloids;  and  it  is  per- 
fectly safe  to  apply  the  conclusions  won  in  this  fashion 
to  the  related  problems  of  histology  and  biology.  Com- 
parative experiments  of  this  type  have  been  made  in 
masterly  fashion,  for  example,  by  G.  MANN.1  Obviously 
even  the  normal  microscopic  structure  of  living  tissues  is 
the  result  of  changes  in  state  of  the*  biocolloids,  wherefore 
detailed  study  of  these,  such  as  is  presented  by  protein  sols 
in  a  test  tube  or  on  a  slide,  can  in  this  fashion  be  used  for 
the  interpretation  of  the  ways  and  means  by  which  normal 
structure  is  produced. 

§24. 

These  and  associated  colloid-chemical  studies  serve  in 
this  way  to  contribute  to  a  science  of  which,  I  admit,  I 
speak  with  reluctance,  even  though  it  is  the  crown  of  all 

1  G.  MANX,  Physiological  Histology,  Oxford,  1002;  see  the  review  of  this 
book  in  Koll.-Zoitsehr.,  2,  153  (1007). 


170  COLLOID   CHEMISTRY 

biology.  I  refer  to  synthetic  biology,  the  science  of  the 
artificial  production  of  living  things.  Since  the  synthetic 
production  of  urea  by  LIEBIG  and  WOHLER,  we  have  been 
familiar  with  a  synthetic  biochemistry.  It  is  today  an  easy 
matter  to  produce  in  the  laboratory  substances  and  re- 
actions which  are  commonly  seen  only  in  living  organisms. 
By  comparison  we  are  still  much  in  the  dark  regarding  a 
sister  science,  that  of  synthetic  biophysics.  Even  when  we 
succeed  in  producing  ameboid  movements  in  drops  of 
liquid  or  in  colloid  mixtures,  or  when  we  discover  methods 
whereby  non-living  matter  can  be  made  to  build  protec- 
tive coverings  for  itself,  to  exercise  choice  in  the  taking 
up  of  nutritive  materials,  we  incline  to  call  these  analogies 
to  biological  processes  " imitations"  of  the  processes  and 
thus  to  cheapen  the  value  which  we  set  on  them.  But  such 
experiments  are  experiments  in  synthetic  biophysics  and  of 
exactly  the  same  significance  as  the  synthesis  of  urea  or  ca- 
talysis by  colloid  metals  for  synthetic  biochemistry. 

Like  the  chemistry,  so  must  the  physics  of  organized 
substance  be  analyzed  into  unit  processes  and  through 
gradual  rebuilding  from  these  be  resurrected  into  a  syn- 
thetic biology.  Trustworthy  results  will,  of  course,  be 
obtained  only  through  systematic  study.  It  would  be 
most  unscientific,  for  example,  to  call  certain  precipitates 
primitive  organisms  just  because  they  look  like  such.  A 
synthetically  produced  organism  must,  naturally,  show  all 
the  fundamental  characteristics  of  organized  matter  at 
one  and  the  same  time.  But  in  spite  of  the  great  dis- 
tance still  to  be  traversed  before  such  a  goal  is  attained, 
there  can  be  no  doubt  that  through  a  proper  combination 
of  individual  chemical  and  physical  processes  of  the  types 
observable  in  organisms,  the  attempt  to  reach  such  a  goal 
represents  an  entirely  scientific  problem.  In  the  still 
much-neglected  biophysics  of  the  colloids  there  is  already 
at  hand  a  wealth  of  suggestive  material. 


SCIENTIFIC  APPLICATIONS  171 

§25. 

I  must  in  conclusion  give  you  a  hasty  view  of  the  appli- 
cations of  colloid  chemistry  to  medicine.  Obviously  the 
number  of  possible  applications  here  is  just  as  great  as  in 
the  biology  of  the  normal  organism,  for  pathological  changes, 
too,  take  place  only  in  that  colloid  foundation  in  which 
all  normal  life  processes  occur.  Just  as  normal  causal 
biology  must  be  edited  —  must  be  rewritten,  in  fact  —  in 
the  terms  of  colloid  chemistry,  even  so  must  pathology  be 
rewritten.  Time  does  not  permit  me  to  enter  into  many 
details,  but  a  few  interesting  examples  will  illustrate  my 
point. 

Closely  associated  with  the  general  problem  of  how  an 
organism  holds  its  normal  water  content  is  that  of  the  ways 
and  means  by  which  it  holds  more  than  this,  as  is  the  case 
in  the  pathological  phenomena  of  edema  and  its  various 
clinical  subheadings.  As  in  the  case  of  the  normal  ab- 
sorption of  water  by  the  normal  organism,  the  tissue  col- 
loids again  play  a  chief  role  in  this  pathological  problem 
as  demonstrated  in  the  fundamental  investigations  of 
MARTIN  H.  FiscHER.1  Changes  in  the  water-holding  ca- 
pacity of  the  tissue  colloids  are  responsible  for  edema  and 
an  abnormal  production  or  accumulation  of  acids  in  the 
involved  tissues  again  appears  to  be  the  main  agent  favor- 
ing the  swelling,  even  though  it  is  possible  that  the  hy- 
drating  effects  of  other  substances  like  the  proteolytic 
ferments  (W.  GIBS)  may  also  act  in  this  direction.  An 
abnormal  production  or  retention  of  acids  can  be  assumed 
or  proved  to  be  the  primary,  etiologic  cause  in  most  cases 
of  edema.  Acids  are  produced,  for  example,  whenever  the 
normal  processes  of  oxidation  are  inhibited,  as  through 
the  presence  of  different  poisons,  through  a  shutting  off 
of  the  circulation  (passive  congestion),  through  anatomical 
changes  in  the  organs  necessary  for  the  maintenance  of 

1  See  the  numerous  papers  of  MARTIN  H.  FISCHER  and  his  collaborators 
in  the  Kolloid-Zeitschrift  and  the  Kolloidchemische  Beihefte  as  well  as  his 
(Edema  and  Nephritis,  second  edition,  New  York,  1915. 


172  COLLOID  CHEMISTRY 

a  proper  circulation,  or  when  in  consequence  of  a  flea  bite 
or  a  bee  sting  formic  acid  is  introduced  locally  into  a  tissue. 
Experimentally  " artificial  flea  bites"  can  be  produced  very 
nicely  by  pricking  a  gelatin  plate  with  a  needle  dipped  in 
formic  acid  and  then  placing  the  gelatin  plate  in  a  little 
water  (demonstration).1 

The  correctness  of  the  view  that  the  water-holding  prop- 
erty of  the  colloids  and  changes  in  their  state  determine 
both  the  normal  and  the  pathological  water  content  of 
tissues  can  also  be  demonstrated  in  the  following  fashion 
(demonstration).  I  have  left  untouched  the  experimental 
apparatus  with  which  I  showed  you  the  influence  of  elec- 
trolytes upon  the  swelling  of  gelatin.  I  have  merely  placed 
in  the  different  solutions  beside  the  gelatin  discs,  whole 
organs,  sheep  eyes,  frog  legs,  etc.,  and  have  left  them 
there  for  a  number  of  hours.  If  you  will  look  at  these 
experiments  you  will  note  that  the  influence  of  these  dif- 
ferent electrolytes  upon  the  swelling  of  these  organs  par- 
allels completely  the  influence  of  these  same  substances 
upon  the  swelling  of  the  gelatin  discs.  You  note  that  in 
the  acid  and  in  the  alkali  there  is  an  enormously  greater 
increase  in  the  size  of  the  organs  than  in  the  pure  water  - 
in  other  words,  they  have  absorbed  more  water  in  the 
acids  and  alkali  than  have  the  organs  left  in  the  pure  water. 
On  the  other  hand,  where  a  proper  salt  has  been  added, 
a  distinct  shrinkage  has  occurred  (Fig.  43).  When  we  touch 
the  organs  which  have  swelled  in  the  acid  we  become  con- 
scious of  the  same  feeling  which  edematous  organs  give  us. 

In  passing  let  me  emphasize  that  just  as  the  addition 
of  salt  to  gelatin  or  fibrin  swelling  in  the  presence  of  an 
acid  reduces  the  amount  of  the  swelling  or  suppresses  it 
entirely,  just  so  has  this  principle  been  successfully  em- 
ployed in  the  reduction  of  clinical  forms  of  edema.2 

It  has  been  argued  against  this  colloid-chemical  theory 

1  See  MARTIN  H.  FISCHER,  (Edema  and  Nephritis,  second  edition,  199  and 
602,  New  York,  1915. 

2  See  the  papers  and  books  of  MARTIN  H.  FISCHER  and  his  collaborators 
—  especially  the  newer  ones  in  the  Kolloidchemische  Beihefte,  4,  343  (1913). 


SCIENTIFIC  APPLICATIONS 


173 


of  edema  that  it  does  not  suffice  to  explain  how  the  ex- 
cessive accumulations  of  fluid  which  are  often  found  be- 
tween the  cells  and  in  the  body  cavities  are  to  be  explained. 
But  this  phenomenon  which  we  are  wont  to  see,  particu- 
larly in  the  latter  stages  of  edema,  is  also  easily  explained 
colloid-chemically.  The  spontaneous  secretion  of  such 
liquids,  which,  as  you  know,  are  often  rich  in  albumin, 


Intensely  (yellowish)  white 


ellowish)  white 
liite  nucleous 


FIG.  43.  —  Swelling  of  sheep  eyes  according  to  MARTIN  H.  FISCHER. 
(a)  the  normal  eye  (c)  in  HC1  plus  Mg(NO3)2 

(6)  inHCl  (d)  in  HC1  plus  FeCla 

is  the  analogue  of  what  we  call  syneresis  in  colloids  and 
may  be  expected  to  appear  in  particularly  marked  form 
whenever  the  gels  from  which  they  are  squeezed  off  are 
particularly  rich  in  water.  As  I  pointed  out  before,  the 
amount  of  fluid  thus  squeezed  off  by  any  hydrophilic  col- 
loid like  gelatin  increases  with  increase  in  the  water  con- 
tent of  the  gel. 


174  COLLOID  CHEMISTRY 

Inspection  of  an  eye  which  has  been  permitted  to  swell 
in  an  acid  shows  the  eye  to  be  in  a  state  which  clinically 
we  would  call  glaucomatous.  The  cornea  is,  moreover, 
steamy  or  opaque  (Fig.  43).  In  the  terms  of  colloid 
chemistry  we  deal  with  an  increased  water  absorption  by 
some  of  the  ocular  colloids  while  the  clouding  represents 
an  acid  coagulation  of  a  second  group  of  the  biocolloids. 
In  the  case  of  this  second  group  the  acid  concentrations 
employed  do  not  bring  about  an  increased  but  rather  a 
decreased  hydration  and  coagulation  of  the  involved  ma- 
terials. Since  our  tissues  represent  a  mixture  of  very 
different  types  of  colloids,  this  double  effect  is  readily 
intelligible  colloid-chemically. 

§26. 

As  also  shown  by  MARTIN  H.  FISCHER,  these  combined 
swelling  and  coagulation  processes  also  occur  in  many  cases 
of  nephritis.1  The  method  of  handling  these  cases  thera- 
peutically  through  the  introduction  of  salt  solutions  — 
more  particularly  of  solutions  alkaline  in  nature  —  as  has 
been  done  with  the  greatest  success  since  FISCHER'S  ex- 
periments, is  also  based  upon  the  colloid-chemical  activity 
of  these  salts.  Such  salts  inhibit  the  swelling  and  coagu- 
lating effects  of  the  acids  formed  in  the  pathologically 
affected  tissues  just  as  they  decrease  their  effects  upon 
simple  mixtures  of  colloids  in  a  test  tube. 

I  can  only  mention  in  passing  that  inflammation  has 
also  been  discussed  from  a  colloid-chemical  point  of  view,2 
that  the  unknown  substance  which  is  associated  with 
goiter3  is  undoubtedly  colloid  in  nature,  that  the  immune 
reactions  in  their  mutual  adsorptions  and  precipitations 
offer  a  field  full  of  unlimited  possibilities  for  the  appli- 
cations of  colloid  chemistry,  that  promising  beginnings  of 

1  See  MARTIN  H.  FISCHER,  (Edema  and  Nephritis,  2nd  Ed.,  518,  New 
York,  1915;  as  well  as  Kolloidchem.  Beihefte,  4,  343  (1913). 

2  A.  OSWALD,  Zeitschr.  f.  exp.  Pathol.  u.  Therapie,  8,  226  (1910);  a  review 
is  found  in  Koll.-Zeitschr.,  9,  251  (1911). 

3  See  E.  BIRCHER,  Ergebnisse  der  Chirurgie  u.  Orthopadie,  6,  133;  Zeitschr. 
f.  exper.  Pathol.,  9,  etc. 


SCIENTIFIC  APPLICATIONS  175 

a   colloid-chemical   theory   of  narcosis   have  been  made/ 
etc. 

I  would  also  point  out  that  a  whole  series  of  inorganic 
colloids  is  now  being  used  therapeutically  —  colloid  sul- 
phur in  skin  diseases,  colloid  mercury  and  mercurial  salts 
in  syphilis,  colloid  nickel  in  meningitis,  colloid  silver  for 
the  antiseptic  treatment  of  wounds  and  in  the  manage- 
ment of  infectious  diseases  like  gonorrhea  and  ophthalmia. 
In  ophthalmology  colloid  silver  has  almost  completely  dis- 
placed silver  nitrate.  A  very  recent  application  of  colloid 
chemistry  is  seen  in  the  use  of  colloid  palladium  oxid  in 
obesity.2  The  treatment  consists  of  the  hypodermic  in- 
jection of  the  material  into  the  fatty  areas  which  are  to 
be  reduced  in  size  and  the  injection,  I  am  happy  to  state, 
is  said  to  be  not  only  not  unpleasant  but  actually  associated 
with  pleasant  sensations. 

§27. 

With  this  I  must  end  the  list  of  the  applications  which 
have  been  made  of  colloid  chemistry  to  neighboring  sciences. 
I  do  it  with  the  hope  that  I  may  have  convinced  you  of 
the  inadequacy  of  any  such  lecture  as  today's  to  portray 
the  countless  applications  that  have  thus  been  made  or 
can  still  be  made.  I  know  that  every  one  of  you  could, 
from  the  special  fields  of  your  particular  endeavors,  at 
once  state  a  problem  which  might  be  investigated  from  a 
colloid-chemical  point  of  view  and  which  I  have  not  at 
all  touched  upon.  The  day  seems  already  here  when  no 
one  speaker  can  by  himself  get  even  an  approximately 
complete  view  of  the  whole  field  comprised  under  this 
heading  of  the  applications  of  colloid  chemistry  to  science. 

1  See  S.  LOEWE,  Biochem.  Zeitschr.,  67,  161  (1913),  where  references  to 
the  literature  may  be  found. 

2  See  M.  KAUFFMANN,  Miinchener  Medizin.  Wochenschr.,  525  (1913);  a 
monograph  discussing  the  results  obtained  by  this  method  is  shortly  to  appear 
from  the  press  of  TH.  STEINKOPFF  of  Dresden. 


T. 

SOME  TECHNICAL  APPLICATIONS  OF 
COLLOID  CHEMISTRY. 


FIFTH  LECTURE. 

SOME  TECHNICAL  APPLICATIONS  OF 
COLLOID  CHEMISTRY. 

I  SHALL  in  this  last  lecture  survey  the  applications  of 
colloid  chemistry  to  some  technical,  industrial  and  prac- 
tical problems.  You  might  begin  by  asking  whether  there 
is  any  purpose  in  spending  a  whole  hour  in  discussing 
these  things.  The  concept  of  the  colloids  has  become  a 
familiar  one  only  recently  and  so  it  might  be  concluded 
that  the  teachings  of  the  colloid  chemist  find  application 
in  these  practical  fields  only  along  narrow  and  specialized 
lines.  Have  enough  and  sufficiently  important  applica- 
tions of  colloid  chemistry  to  technology  really  been  made 
to  justify  spending  a  whole  hour  upon  the  subject?  Let 
me  in  answer  ask  you  to  accompany  me  for  a  moment. 

§1. 

The  clothes  you  wear,  be  they  wool,  cotton  or  silk,  are 
animal  or  plant  gels.  They  are  dyed  with  colors  which,  in 
many  instances,  as  the  indigos  and  the  blacks,  are  colloid 
in  type.  In  the  process  of  dyeing,  adsorption  and  other 
colloid-chemical  reactions  take  place  between  the  colloid 
substrates  of  the  fabric  and  the  colloid  dyes  which  color 
them.  The  leather  of  your  shoes  is  an  animal  gel,  closely 
related  in  its  general  properties  to  that  prototype  of  the 
colloids,  gelatin.  Leather  is  tanned  with  substances  of 
which  the  majority  are  colloids,  and  the  whole  process  of 
tanning  is  punctuated  with  the  colloid  phenomena  of  hy- 
dration,  dehydration  and  adsorption.  The  wood  of  the 
chairs  in  which  you  rest  is  made  of  cellulose,  which  in  all 

179 


180  COLLOID  CHEMISTRY 

its  various  forms  is  colloid  in  nature.  The  colloid  swelling 
of  wood,  as  I  emphasized  earlier,  was  used  by  the  old 
Egyptians  to  aid  their  quarrying  of  stone.  The  woods 
of  your  chairs  are  held  together  by  glue  or  with  metals. 
You  already  know  glue  to  be  a  colloid,  but  it  may  surprise 
you  to  learn  that  colloid  chemistry  has  much  to  say  in 
metallurgy  and  that  steel,  for  instance,  is  a  colloid  solid 
solution.  We  shall  return  to  this  question.  The  paper 
upon  which  you  write  is  essentially  cellulose,  in  other 
words,  again  colloid.  It  has  been  given  a  body  by  being 
mixed  with  water-glass,  with  rosin  or  some  similar  material, 
in  other  words,  with  various  colloids.  The  ink  in  your 
fountain  pens  is  probably  also  colloid  if  it  is  the  ordinary 
iron  tannate,  and  colloid,  too,  is  the  hard  rubber  of  your 
pen  holders,  prepared  from  that  notoriously  colloid  mother 
substance,  soft  rubber. 

I  could  continue  this  list  indefinitely,  pointing  in  this 
manner  to  one  colloid  after  another  in  your  immediate 
surroundings  and  belonging  to  the  things  of  your  every- 
day life.  Perhaps  you  think  —  perhaps  since  yesterday's 
lecture  you  think  you  know  —  that  I  am  possessed  of  a 
colloid  mania  because  I  see  colloids  everywhere.  Let  me 
admit  that  I  do  see  colloids  everywhere,  only  I  do  not 
believe  that  because  of  this  I  must  be  adjudged  insane. 
It  is  simply  a  fact  that  colloids  constitute  the  most  universal 
and  the  commonest  of  all  the  things  we  know.  We  need  only 
to  look  at  the  sky,  at  the  earth,  or  at  ourselves  to  dis- 
cover colloids  or  substances  closely  allied  to  them.  We 
begin  the  day  with  a  colloid  practice  —  that  of  washing  - 
and  we  may  end  it  with  one  in  a  bedtime  drink  of  colloid 
tea  or  coffee.  Even  if  you  make  it  beer,  you  still  consume 
colloid.  I  make  these  remarks  in  full  earnest  and  with  the 
request  that  if  I  do  not  prove  my  assertions  to  your  satis- 
faction, you  challenge  me  in  the  matter. 

These  facts  leave  no  doubts  in  our  minds  as  to  the  wealth 
and  variety  of  the  possible  technical  and  practical  appli- 
cations of  colloid  chemistry.  We  only  become  conscious 


TECHNICAL  APPLICATIONS  181 

again  of  a  great  difficulty  in  making  a  proper  choice  of 
illustrations  from  the  wealth  of  material  before  us. 

To  this  difficulty  come  two  others.  Colloid  chemistry 
as  a  systematically  studied  science  is  still  very  young.  It 
cannot  therefore  be  expected  that  any  conscious  applica- 
tion of  colloid  chemistry  to  technology  has  as  yet  been  made 
in  anything  like  the  degree  possible  or  probable.  Many 
technical  experts  do  not  as  yet  even  know  that  in  their 
every-day  practices  they  are  working  in  colloids  and  that 
they  should,  in  consequence,  employ  the  fruits  of  scientific 
colloid  chemistry  in  their  various  endeavors.  This  fact 
is  often  brought  home  to  the  colloid  chemist  who  enters 
into  discussion  with  practical  men  —  something  which,  by 
the  way,  every  scientist  should  do  as  often  as  possible. 

I  remember  a  conversation  with  a  brick  manufacturer 
who  complained  because  two  lots  of  clay  which  were  alike 
chemically,  yielded  products  of  very  different  qualities. 
I  expressed  the  conviction  that  a  difference  in  the  colloid 
state  of  the  clays  was  probably  responsible.  "  Colloid 
state,  what  do  you  mean  by  that?"  he  answered.  Our 
conversation  then  turned  to  a  discussion  of  colloids  and  he 
thus  heard  for  the  first  time  of  the  fundamental  properties 
of  materials  in  which  he  had  worked  for  decades.  Need- 
less to  add  he  became  an  enthusiast  in  colloid  chemistry 
and  I  doubt  not  that  he  is  now  a  regular  subscriber  to  the 
Kolloid-Zeitschrift . 

What  I  have  said  of  the  brick  manufacturer  is  true  of 
many  other  branches  of  industry.  In  many  of  them  even 
that  first  of  steps  needs  yet  to  be  taken,  namely,  that  of 
recognizing  the  colloid  nature  of  the  materials  being  worked 
upon  and  the  colloid  nature  of  the  processes  being  used  in 
manufacture.  The  details  of  technical  procedure  need  to 
be  rewritten  in  the  terms  of  colloid  chemistry.  Let  it  be 
clearly  understood  that  this  does  not  mean  a  mere  restate- 
ment of  facts  and  problems  in  different  terms.  When  I 
say  that  rubber  or  cellulose  is  a  solid  colloid  or  that  this 
or  that  technical  process  represents  an  adsorption  phe- 


182  COLLOID  CHEMISTRY 

nomenon,  I  have  to  assume  full  responsibility  for  such 
statements  and  must  be  able  to  prove  that  the  substances 
concerned  show  the  fundamental  properties  of  the  colloid 
state  or  that  the  processes  declared  adsorptive  hi  nature 
obey  the  adsorption  laws.  Such  new  definition  in  the 
terms  of  colloid  chemistry  is  by  no  means  always  as  simple 
as  might  at  first  appear.  I  beg  you  to  remember  this 
when  in  the  following  discussion  I  am  often  merely  able 
to  state  that  this  is  a  technically  interesting  colloid  as 
shown  by  such  and  such  properties;  that  these  processes 
are  probably  colloid-chemical  in  nature,  etc.  Complete 
colloid-chemical  analysis  and  an  accurate  distraction  be- 
tween such  processes  in  technological  practice  as  are  colloid- 
chemical  in  nature  and  such  as  are  not  has  been  found 
possible  thus  far  in  only  isolated  instances.  There  is  a 
wealth  of  work  to  be  done  here,  interesting  not  only  from 
a  scientific  standpoint  but  from  a  practical  one  as  well. 

§2. 

Permit  me  after  this  lengthy  introduction  to  enter  at 
once  upon  a  consideration  of  the  use  of  the  inorganic  col- 
loids hi  practice.  The  whole  list  of  elements  finds  use  in 
colloid  form  in  industry;  to  a  few  of  these  elements  I  would 
like  to  call  your  attention. 

An  interesting  and  characteristically  American  product 
consisting  of  an  element  in  colloid  form  is  the  so-called 
ACHESON  graphite.1  I  have  shown  you  this  before  under 
the  name  aquadag  as  a  dispersion  in  an  aqueous  dispersion 
medium.  I  show  it  to  you  again  dispersed  in  a  mineral 
oil  under  the  name  oildag.  These  two  preparations,  which 
are  much  used  as  lubricants,  prove  on  investigation  to 
be  typical  colloids.  This  fact  is  revealed  by  ultramicro- 
scopic  examination,  by  the  migration  of  the  black  phase 
in  the  electric  field,  by  the  precipitation  effects  produced 
through  the  addition  of  acids  or  sodium  chlorid,  etc.  This 

1  I  am  greatly  indebted  to  Dr.  ACHESON  for  a  large  quantity  of  demon- 
stration material. 


TECHNICAL  APPLICATIONS  183 

technically  important  material  serves  also  to  illustrate  my 
remark  that  much  work  is  done  with  colloids  without  the 
workers  being  conscious  that  they  are  working  with  this 
type  of  material.    ACHESON  did  not  begin  with  intent  to 
prepare  colloid  graphite  as  is  evident  from  his  interesting 
addresses   upon   the   history   of   his   discovery.     He   only 
afterwards   became   conscious   of   the   similarity   and   the 
relationships  of  his  preparation  to  colloid  types  of  materials. 
Aside  from  the  fact  that  aquadag,  because  of  its  black 
color  and  accessibility  is  useful  as  material  upon  which  to 
demonstrate  the  fundamental  properties  of  colloids  (diffu- 
sion,  dialysis,   filtration,   electrophoresis,   coagulation,   ad- 
sorption, etc.)  it  is  a  preparation  which  is  of  great  colloid- 
chemical  interest  in  other  directions  as  well.     First  of  all, 
its  method  of  preparation  is  interesting,  of  which  ACHESON 
says  that  he  finds  it  described  in  the  Bible,  or  rather  in 
the  reports  contained  in  the  Bible  of  the  methods  employed 
by  the  Egyptians  in  the  production  of  high-quality  bricks. 
The  Egyptians  used  straw  infusions  and  other  liquids  rich 
in  tannin  in  order  to  obtain  uniform  and  finely  divided 
clay.     In  the  same  fashion  ACHESON  made  use  of  commercial 
tannin  preparations  in  order  to  obtain  a  highly  dispersed, 
stabile  and  concentrated  preparation  of  ground  graphite. 
Just  as  when  colloid  gold  is  made  with  tannin,  so  hi  the 
preparation  of  colloid  clays  and  of  graphite,  the  tannins  act 
as  typical  protective  colloids.     When  graphite  is  ground 
in  the  presence  of  tannin  the  highly  dispersed  and  colloid 
particles  of  graphite  are  encompassed  as  formed  by  the 
strongly  hydrated  tannin  and  so  prevented  from  running 
together  again  into  larger  aggregates.     But  because  of  the 
great  stability  of  the  tannin  toward  electrolytes,  it  now 
becomes  possible  to  evaporate  a  part  of  the  dispersion 
medium  until  a  graphite  paste  is  obtained  without  the 
graphite   settling   out.     The   presence   of   this   protective 
colloid  also  helps  to  keep  the  graphite  finely  divided  when 
the  lubricant  in  practice  is  exposed  to  the  precipitation 
dangers  incident  to  coming  hi  contact  with  electrolytes. 


184  COLLOID  CHEMISTRY 

Another  point  of  interest  is  that  the  lubricating  action 
of  the  graphite  is  intimately  connected  with  its  degree  of 
dispersion.  Even  ordinary,  coarsely  dispersed  graphite  is 
a  good  lubricant,  but  its  effectiveness  is  much  increased 
by  simple  grinding,  when  the  so-called  gredag  is  obtained. 
But  these  coarse  preparations  are  excelled  by  colloid  graph- 
ite. We  see  here  again  a  property,  namely,  lubricating 
activity,  which  increases  steadily  with  increase  in  degree 
of  dispersion.  As  the  technologists  among  you  well  recog- 
nize, we  still  know  very  little  regarding  the  properties 
which  make  a  substance  a  good  lubricant.  Perhaps  a 
closer  study  of  this  relationship  between  lubricating  effect 
and  degree  of  dispersion  in  heterogeneous  lubricating  ma- 
terials may  bring  us  some  light,  which  may  in  its  turn 
tell  us  what  makes  certain  homogeneous  liquids  good 
lubricants  and  others  not. 

§3. 

Other  elements  which  are  used  in  colloid  form  in  techno- 
logical processes  are  seen  in  the  metals.  An  interesting 
and  old  application  of  colloid  chemistry  to  technology  is 
seen  hi  the  use  of  colloid  metals  for  the  production  of 
incandescent  light  filaments.  As  you  know,  the  lighting 
expert  is  constantly  trying  to  bring  the  various  light- 
giving  bodies  used  in  his  lamps  to  as  high  a  temperature 
as  possible,  since  by  this  means  he  shifts  the  relation  of 
the  visible  to  the  invisible,  or  heat  rays  toward  the  side 
of  the  light  rays.  For  this  reason  the  attempt  is  old  which 
would  replace  the  easily  vaporized  carbon  filaments  of  our 
older  lamps  by  the  less  volatile  metallic  filaments  of  tung- 
sten, tantalum,  etc.  These  metals  have,  however,  the 
unwelcome  property  of  great  brittleness  so  that  they  can 
not  be  drawn  into  threads.  To  overcome  the  difficulty 
the  metals  were  used  in  finely  divided  form.  Colloid 
powders  of  the  metals  were  made  into  pastes,  often  through 
the  addition  of  some  hydratable  organic  colloid,  and  the 
pastes  were  then  squeezed  through  fine  openings  (as  in 


TECHNICAL  APPLICATIONS  185 

the  manufacture  of  artificial  silk),  and  in  this  way  very 
fine  threads  of  the  metals  were  obtained.  I  am  able  to 
show  you  here  some  hair-like  threads  of  tungsten  prepared 
in  this  manner  (demonstration).1 

Of  special  interest  is  the  method  of  KUZEL  for  the  prep- 
aration of  such  colloid  metals  in  powder  or  paste  form. 
Prolonged  grinding  alone  suffices  to  convert  a  low  percent 
of  the  metal  into  the  form  of  the  colloidally  divided  powder. 
I  show  you  some  tungsten  powder  prepared  in  this  way. 
Thrown  upon  a  filter  you  observe  that  it  runs  through 
as  soon  as  some  distilled  water  is  poured  upon  it  (demon- 
stration). This  mechanical  grinding  takes  much  time  and 
is  very  expensive.  It  is  simpler  and  cheaper  to  make  a 
colloid  mud  of  the  metal  by  the  method  of  KUZEL.  In 
this  the  powdered  metal  is  successively  and  repeatedly 
treated  with  acids  and  alkalies  interspersed  with  washings 
in  distilled  water.  The  theory  of  the  process  is  about 
as  follows. 

In  a  dilute  acid  the  surface  layer  of  the  coarsely  dispersed 
particles  of  metal  goes  into  solution  in  finer  form,  yielding 
a  smaller  gram.  This  reduces  a  part  of  the  powder  to 
colloid  dimensions.  If  the  acid  were  allowed  to  act  too 
long,  the  colloid  thus  formed  would  dissolve  completely 
and  disappear.  The  acid  must  therefore  be  neutralized 
and  washed  away,  while  the  colloid  which  has  been  formed 
is  separated  by  precipitation  or  filtration  from  the  still 
coarsely  divided  material.  By  repeating  the  process,  an- 
other part  of  the  metal  is  brought  into  the  colloid  state, 
the  whole  procedure  being  repeated  time  after  time  until 
all  the  metal  is  gotten  into  colloid  form. 

We  have  learned  recently  how  to  temper  metals,  how  to 
make  alloys  with  others  and  thus  how  to  get  such  metals 
as  we  are  discussing  into  ductile  form.  As  we  make  progress 
along  such  lines,  we  may  expect  to  see  the  colloid  method 
discussed  above  displaced  by  these  simpler  procedures. 

1  Colloid  incandescent  lamp  filaments  were  kindly  placed  at  my  disposal 
by  the  Chemische  Fabrik  VON  HEYDEN. 


186  COLLOID  CHEMISTRY 

§4. 

Colloid  metals  and  colloid  metallic  compounds  are  much 
used  to  give  color  to  various  materials.  Ruby  glass  owes  its 
red  color  to  the  presence  of  colloid  gold.  I  show  you  three 
specimens  which  are  "  solid  solutions "  of  gold  in  three 
very  different  and  characteristic  degrees  of  dispersion 
(demonstration).1  The  first  is  an  almost  clear  and  but 
slightly  yellow  mass  of  glass.  This  is  obtained  immedi- 
ately after  dissolving  the  solid  gold  salt  in  the  glass.  There 
is  obtained  in  this  way  a  molecularly  dispersed  solution 
of  the  gold  in  the  glass,  and  one  which,  in  consequence, 
is  ultramicroscopically  empty.  The  second  preparation  is 
the  ordinary  ruby  glass  in  which  the  gold  is  contained  in 
a  colloid  state.  The  third  specimen  is  deep  blue  by  trans- 
mitted light  and  orange  brown  by  reflected  light.  The 
specimen  is  also  distinctly  turbid.  It  springs  from  a  failure 
in  glass  manufacture  in  that,  presumably  through  a  too  long 
heating  of  the  glass,  a  coagulation  of  the  red  gold  particles 
to  the  more  coarsely  dispersed  blue  particles  has  taken  place 
—  just  such  a  change  as  I  showed  you  in  an  aqueous  dis- 
persion medium  when  I  coagulated  the  red  gold  (produced 
through  reduction  of  gold  chlorid  by  tannin)  to  blue  gold 
through  the  addition  of  acid.  These  same  facts  as  illus- 
trated in  the  case  of  glass  prove  of  what  little  importance 
is  the  kind  of  dispersion  medium  and  how  much  depends 
upon  the  degree  of  dispersion  in  determining  the  varia- 
tions in  color  in  this  substance. 

Silver  colloidally  dispersed  in  glass  makes  it  yellow  or 
brown.  Selenium  colors  it  beautifully  red  or  violet.  The 
colloid  metallic  hydroxids  also  impart  to  glass  some  beauti- 
ful colors,  as  illustrated  in  the  production  artificially  of 
the  precious  stones.  The  artificial  rubies  owe  their  color 
presumably  to  a  colloid  chromium,  as  do  the  artificial 
alexandrites,  etc. 

1  Different  specimens  of  gold  ruby  glass  were  kindly  placed  at  my  dis- 
posal by  POPPER  AND  SONS  of  New  York. 


TECHNICAL  APPLICATIONS  187 

§5. 

An  interesting  illustration  of  color  due  to  a  colloid  ele- 
ment is  seen  in  the  case  of  ultramarine.  There  still  rages 
an  old  debate  concerning  the  causes  for  the  color  in  this 
mixture  of  different  silicates,  borates,  etc.,  with  sulphur 
or  sulphur  compounds.  Even  recently,  nothing  short  of 
desperate  efforts  have  been  made  to  explain  the  color  of 
this  dye  substance  on  the  basis  of  its  "  chemical  constitu- 
tion." I  say  desperate  because  not  only  are  the  quantita- 
tive relationships  found  in  the  different  ultramarines  totally 
different,  but  on  heating  the  normal  ultramarine,  uncolored, 
grey,  yellow,  red,  blue  and  even  green  ultramarines  can  be 
produced,  as  shown  in  these  specimens  (demonstration). 
On  the  basis  of  differences  in  chemical  constitution  we 
would  have  to  assume  that  each  of  these  different  colors 
represented  a  different  chemical  compound. 

What  we  observe  is  entirely  analogous  to  what  we  dis- 
cussed previously  when  dealing  with  the  photohaloids. 
We  can  produce  blue  and  green  solutions  of  sulphur  by 
simply  introducing  this  element  into  molten  sodium  chlo- 
rid,  into  a  borax  bead,  into  liquid  ammonia  or  into  hot 
organic  liquids  like  glycerin.1  These  facts  render  it  most 
improbable  that  ultramarine  is  blue  because  of  the  exist- 
ence in  it  of  a  specific  blue  sulphur  compound.  We  have 
therefore  come  to  the  conclusion  that  ultramarine  repre- 
sents a  solid  solution  of  highly  dispersed  sulphur.  The 
degree  of  dispersion  may  oscillate  between  molecular  and 
colloid  dimensions,2  and  as  this  happens  the  different  colors 
discussed  above  which  represent  different  degrees  in  the 
dispersion  of  the  element  sulphur  are  produced. 

Of  the  many  facts  which  confirm  this  view,  I  would  like 

1  See  Kolloidchem.  Beih.,  2,  449  (1911). 

2  The  assumption  that  we  have  to  do  with  a  solid  solution  of  elementary 
sulphur  in  the  case  of  the  ultramarines  was  first  made  by  J.  HOFFMANN  [see, 
for  example,  Koll.-Zeitschr.,  10,  275  (1912)],  but  that  we  deal  with  solid 
solutions  possessed  of  a  colloid  degree  of  dispersion  or  of  one  approximating 
these  dimensions,  seems  first  to  have  been  expressed  by  me  [Kolloidchem. 
Beih.,  2,  449  (1911);  also  Koll.-Zeitschr.,  12,  61  (1913)]. 


188  COLLOID   CHEMISTRY 

to  emphasize  the  analogy  between  the  production  of  ultra- 
marine and  the  production  of  ruby  glass,  of  blue  rock  salt, 
etc.  In  making  ultramarine  the  necessary  salts  and  the 
sulphur  are  melted  together  at  a  high  temperature.  This 
yields  the  greyish  white  or  yellowish  "  mother  of  ultra- 
marine." This  product  is  then  reheated,  cooled  and  re- 
heated again,  just  as  in  the  case  of  ruby  glass,  until  the 
requisite  color  is  obtained.  The  original  product  is  ob- 
viously a  molecularly  dispersed  solution,  the  particles  of 
which,  through  reheating,  are  permitted  to  condense  to 
colloid  dimensions.  Support  for  the  correctness  of  this 
view  may  be  found  in  mineralogy.  Mineralogists  are 
familiar  with  a  complex,  sulphur-rich,  silicate  compound 
known  as  hauynite  which  appears  in  different  colors  rang- 
ing from  colorless  to  green  and  blue.  It  has  been  shown * 
that  the  colorless  varieties  may  be  colored  blue  or  green 
by  heating  them  with  sulphur  in  a  closed  tube,  an  experi- 
ment entirely  analogous  to  the  production  of  blue  rock 
salt  by  heating  this  with  metallic  sodium. 

There  exist  reasons  for  believing  that  in  the  colors  of 
many  of  the  so-called  sulphur  dyestuffs  (dyes  produced 
by  melting  together  sulphur  and  different  organic  com- 
pounds in  the  presence  of  alkali 2)  we  have  also  to  do  with 
similar  solid  solutions  of  highly  dispersed  sulphur.  There 
certainly  exists  little  hope  of  explaining  their  colors  on  the 
basis  of  chemical  constitution. 

Let  me  hi  passing  emphasize  that  the  alkali  used  in  the 
preparation  of  either  the  inorganic  or  the  organic  sulphur 
systems  tends  to  increase  their  degrees  of  dispersion.  It 
has,  in  other  words,  a  peptizing  or  stabilizing  influence. 

As  you  doubtless  know  already,  many  of  the  native  gels 
are  used  directly  as  coloring  substances.  I  need  but  men- 
tion the  hydrated  or  burnt  iron  hydroxid  gels  (terra  di 
siena,  umber,  yellow  and  red  ochre,  etc.). 

1  See  Koll.-Zeitschr.,  12,  62  (1913);    also  NAUMANN-ZIRKEL,  Lehrbuch 
der  Mineralogie,  665. 

2  See  O.  LANGE,  Die  Schwefelfarbstoffe,  ihre  Herstellung  usw.,  Leipzig, 
1912,  as  well  as  the  review  of  this  volume  in  the  Koll.-Zeitschr.,  cited  in  the 
previous  footnote. 


TECHNICAL  APPLICATIONS  189 

§6. 

Colloid  chemistry  finds  many  applications  in  photography 
and  the  various  graphic  arts.  I  have  already  touched  upon 
the  fact  that  many  inks  like  the  old  iron  tannate  inks  and 
india  ink  are  colloid  solutions.  Printing  inks  are  given 
their  proper  body  by  being  mixed  with  colloids.  Gelatin 
and  other  colloid  mixtures  are  used  in  different  kinds  of 
color  printing,  etc.  Not  only  does  the  colloid  chemistry 
of  the  photohaloids  play  a  great  role  in  photography,  but 
many  other  colloid-chemical  processes  are  encountered  in 
the  manufacture  of  dry  plates.  This  is  illustrated  in  the 
" ripening"  of  the  photosensitive  emulsions  until  an  opti- 
mal size  of  " granule"  is  obtained,  and  in  the  different 
methods  employed  for  the  developing,  intensifying  and 
printing  of  dry  plates.  To  those  of  you  who  are  interested 
in  these  subjects  I  suggest  a  perusal  of  the  many  contri- 
butions of  LUPPO-CRAMER,  R.  E.  LIESEGANG  and  others.1 

§7. 

Of  other  fields  in  which  colloid  chemistry  plays  an  im- 
portant part  I  would  emphasize  those  of  ceramics  and  the 
hydraulic  cements.  The  earths  and  clays  are  in  great 
measure  typical  gels,  consisting,  as  they  do,  so  largely  of 
aluminium  silicate  and  iron  hydroxid,  admixed  with  or- 
ganic colloids  of  the  type  of  the  humus  acids.  The  plas- 
ticity of  the  ceramic  clays  is  dependent  in  large  part  upon 
their  colloid  content;  and  their  changes  with  time,  the 
effects  of  added  straw  infusions,  of  treatment  with  am- 
monia, etc.,  are  all  processes  intended  either  to  increase 
their  absolute  colloid  content  or  to  increase  the  peptiza- 
tion  or  hydration  of  this  content. 

The  effect  of  alkalies  upon  ceramic  clays  is  so  great  and 
plays  so  important  a  role  in  modern  industry  that  it  "has 
produced  a  revolution  in  ceramics,"  as  one  of  the  first 
authorities  in  this  field  has  expressed  it.  Addition  of 

1  See  the  literature  cited  in  the  first  footnote  on  page  132. 


190  COLLOID  CHEMISTRY 

alkalies  tends  to  " liquefy"  the  clays.  A  stiff  clay  when 
treated  with  proper  amounts  of  alkali  loses  its  stiffness 
and  changes  to  a  liquid.  On  the  other  hand,  dry  clay 
mixed  from  the  beginning  with  relatively  small  amounts 
of  water,  but  containing  some  alkali,  yields  a  fluid  or  semi- 
fluid mass.  The  technical  importance  of  these  findings 
resides  in  the  fact  that  by  such  means  much  smaller 
amounts  of  water  need  to  be  used  and  so  a  faster  drying 
of  the  clay  moulds  is  obtained  preparatory  to  firing.  But 
in  this  fashion  the  much-feared  cracking  and  deformation 
ordinarily  incident  to  the  first  drying  is  also  greatly  re- 
duced. These  alkalinized  clays  can,  moreover,  be  poured 
and  in  this  manner  large  objects  like  bathtubs  can  be  made 
much  more  easily  than  by  the  older  moulding  process.1 

The  explanation  of  how  the  alkali  produces  its  effects 
is  somewhat  complicated;  in  it  at  least  three  to  four  dif- 
ferent colloid-chemical  processes  overlap  each  other.  The 
clay,  an  electro-negative  colloid,  is  peptized  by  the  addi- 
tion of  small  but  definite  amounts  of  alkali.  This  puts 
it  into  a  more  highly  dispersed  state,  an  effect  entirely 
similar  to  that  observed  when  other  negative  colloids,  like 
those  of  the  metals,  are  treated  in  this  manner.  Second, 
a  swelling  is  produced  in  the  particles  of  aluminium  sili- 
cate, a  process  which  apparently  attains  its  optimum  at 
a  somewhat  higher  concentration  of  the  alkali  than  that 
needed  for  peptization.  To  bring  about  this  swelling  effect, 
time  is  needed.  Third,  the  alkali  affects  the  organic  col- 
loids like  tannin,  humus  acids,  etc.,  constantly  present 
under  industrial  circumstances.  These  substances  are  also 
peptized  in  low  concentrations  of  the  alkali,  while  larger 
amounts  not  only  leach  them  out,  but  bring  them  into  a 
molecularly  dispersed  condition.  In  this  way  they  lose 
their  importance  as  " protective"  hydrated  emulsoids.  The 
three  effects  work  side  by  side,  so  that  when  the  element 

1  A  discussion  of  the  literature  and  of  the  patents  covering  the  use  of 
alkalies  in  ceramics  may  be  found  in  J.  K.  NEUBERT,  Kolloidchem.  Beih., 
4,  261  (1913). 


TECHNICAL  APPLICATIONS  191 

of  time  is  also  added  a  fairly  complicated  picture  results. 
In  short  periods  of  time,  peptization  and  increase  in  vis- 
cosity tend  to  nullify  each  other.  It  is  usually  held  that 
the  clays  which  on  admixture  with  water  take  longest  to 
settle  out  are  also  the  most  fluid,  but  this  parallelism 
which  is  much  used  analytically  may  be  lost  as  with  more 
time  the  effect  of  the  alkali  brings  about  an  increased 
swelling  of  the  clay  particles.1 


The  technically  important  process  of  setting,  as  observed 
in  the  hydraulic  cements  (cement  and  mortar),  may  be 
defined  chemically  as  a  reaction  between  calcium  and  silicic 
acid  associated  with  a  taking  up  of  considerable  amounts  of 
water.  These  are  the  fundamental  changes  which  occur  in 
the  setting  of  all  the  cements  even  though  the  materials  that 
are  mixed  with  the  cements  and  mortars  are  for  various 
practical  purposes  very  different. 

These  fundamental  changes  of  chemical  combination  with 
hydration  may  already  be  observed  when  calcium  oxid  and 
sand  are  mixed  together  as  in  ordinary  mortar.  In  the  case 
of  Portland  cement,  the  effects  of  admixture  with  calcium 
and  iron  hydroxid  are  superadded.  It  is  not  our  problem 
to  say  how  much  justice  there  is  in  the  various  specific 
chemical  assumptions  which  have  been  made  to  explain  the 
nature  of  these  fundamental  setting  processes.  It  is  widely 
believed  that  there  comes  into  play  a  whole  series  of  different 
and,  in  part,  crystalline  compounds  (mono-,  di-,  tri-calcium 
silicate;  alite,  belite,  celite,  etc.).  I  would  only  emphasize 
that  every  exclusively  chemical  theory  is  unable  to  explain 
the  physical  accompaniments  which  are  so  characteristic  of 
the  setting  process.  Chemical  reactions  associated  with 
hydration,  like  those  observed  in  the  mortars  and  cements, 
take  place  between  many  substances  without  the  reaction 
mixture  developing  the  characteristic  physical  properties 

1  See  the  detailed  discussion  by  JOH.  K.  NEUBERT  cited  in  the  previous 
footnote. 


192  COLLOID  CHEMISTRY 

observed  in  the  mortars  and  cements.  In  the  setting  of 
mortar  or  cement  there  must  take  place  certain  special 
changes  besides  the  chemical  which  are  responsible  for  the 
physical  peculiarities  resulting  from  these  reactions.  Recent 
investigations  have  proved  the  existence  of  such  and  it  has 
been  found  that  we  again  deal  with  colloid-chemical  proc- 
esses.1 

From  microscopic  study  of  the  setting  process,  it  was  long 
known  that  when  cement  is  mixed  with  water,  numerous 
needle-like  crystals  begin  to  form  around  every  cement 
particle,  consisting,  supposedly,  of  calcium  mono-silicate, 
while  larger  and  smaller  hexagonal  crystals,  presumably 
tri-calcium  aluminate  and  calcium  hydrate,  appear  in  the 
interstices.  But  there  appears  also  a  structure  which,  while 
noted  before,  has  had  a  proper  significance  given  it  only 
recently.  There  forms  about  every  cement  particle  a  gel  of 
calcium  silicate,  the  volume  of  which  increases  steadily  during 
the  process  of  setting  until  it  fills  not  only  the  interstices  between 
the  crystalline  needles  but  all  those  between  the  individual 
cement  particles.  It  is  the  formation  of  this  gel  which  is 
undoubtedly  the  most  important  factor  in  the  process  of 
setting  and  which  gives  the  solid  cement  its  specific  physical 
properties.  The  gel  serves  to  bind  together  not  only  the 
individual  crystals  surrounding  a  given  cement  particle,  but 
also  the  crystals  of  neighboring  particles.  Cement  particles 
with  their  crystals  become  imbedded  in  a  common  sheath 
of  gelatinous  substance  (Fig.  44)  and  it  is  this  fact  which 
gives  cement  its  solidity.  Felting  of  the  crystalline  needles 
could  by  itself  not  explain  this  solidity.  The  cement  tends 
to  become  progressively  harder  as  more  and  more  water  is 
taken  from  the  binding  gel  into  the  innermost  layers  of  the 
cement  particles  themselves.  The  gel  which  we  have  been 
discussing  can  be  stained  by  various  dyes  (like  anthrapur- 

1  See  especially  W.  MICHAELIS,  Koll.-Zeitschr.,  6,  9  (1909);  7,  320  (1910); 
S.  KEISERMANN,  Kolloidchem.  Beih.,  1,  423  (1910),  where  references  to  the 
literature  may  be  found;  as  well  as  numerous  shorter  papers  by  P.  ROHLAND 
to  be  found  in  the  Kolloid-Zeitschrift. 


TECHNICAL  APPLICATIONS 


193 


purin)  and  so  be  made  to  stand  out  from  the  rest  of  the 
cement.  Perhaps  you  would  like  to  see  an  ultramicroscopic 
picture  of  this  setting  process.  I  show  you  such  in  Fig. 
45,  in  which  you  may  see  very  distinctly  the  needle-like 
crystals  protruding  from  the  gel  mass. 

If  we  accept  the  production  of  such  a  gel  as  the  char- 
acteristic element  in  the  process  of  setting,  as  has  been 


FIG.  44.  —  Diagram  illustrating  the  changes  incident  to  the  setting  of  cement 
according  to  W.  MICHAELIS.  The  serrated  lines  indicate  the  outlines  of 
the  gel. 

especially  well  insisted  upon  by  W.  MICHAELIS,  we  at  once 
obtain  the  explanation  of  a  whole  series  of  technological 
details.  In  order  that  the  gel  formation  may  be  complete, 
there  must,  of  course,  be  enough  water  present.  It  is  for 
this  reason  that  in  testing  for  the  maximal  rigidity  of  cement 
samples,  it  is  necessary  to  allow  these  to  harden  under  water. 
Of  especial  technical  importance  is  a  regulation  of  the  rate 
of  the  setting.  Setting  is  retarded,  for  example,  through 


194 


COLLOID  CHEMISTRY 


the  addition  of  such  hydrophilic  organic  colloids  as  glue. 
This  is  explained  colloid-chemically  by  finding  that  the  glue 
takes  up  the  water  and  then  gradually  yields  the  water  to 


FIG.  45.  —  Ultramicroscopic  photograph  of  cement  in  the  process  of  setting, 
according  to  H.  AMBRONN.  Observe  the  delicate  crystalline  needles  pro- 
truding from  the  gel  envelopes. 

the  silicate  gel  as  this  forms.  Conversely,  the  setting 
process  is  hastened  by  adding  organic  acids  like  acetic  acid. 
Colloid-chemically  this  means  that  acetic  acid  favors  the 
formation  of  the  silicate  gel. 

Entirely  in  agreement  with  this  colloid-chemical  theory  of 


TECHNICAL  APPLICATIONS  195 

setting  is  the  fact  that  so  simple  a  process  as  the  hydration 
of  plaster-of-paris  also  begins,  according  to  A.  CAVAZZI,  with 
the  formation  of  a  gelatinous  hydrate  of  the  calcium  sulphate 
which  only  subsequently  tends  to  crystallize  out  in  part.1 
That  in  the  setting  of  plaster-of-paris  the  same  relationships 
obtain  as  in  the  setting  of  cement  and  similar  substances,  is 
indicated  by  the  use  of  glue  to  delay  the  setting  of  plaster- 
of-paris. 

§9. 

I  come  next  to  an  especially  important  chapter  in  tech- 
nology, that  of  the  application  of  colloid  chemistry  to 
metallurgy.  I  must  preface  my  remarks  by  saying  that  the 
possibilities  which  colloid  chemistry  holds  for  the  explanation 
of  many  metallurgical  problems  have  hardly,  as  yet,  been 
recognized.  If  in  the  following  I  venture  some  colloid- 
chemical  points  of  view  to  explain  various  metallurgical 
questions  and  you  find  these  not  touched  upon  in  your 
studies  of  the  orthodox  authorities  who  work  in  these  fields, 
please  know  that  I  do  this  only  because  I  am  convinced  that 
the  colloid-chemical  or  dispersoid-chemical  point  of  view  is 
going  to  have  a  remarkable  future  in  metallurgy. 

Problems  which  are  definitely  colloid-chemical  in  nature 
appear  in  the  initial  processes  of  mining.  It  is  well  known, 
for  example,  that  gold  can  only  with  great  difficulty  be 
extracted  from  deposits  which  are  rich  in  various  earths  or 
clays.  The  metal  present  in  these  materials  is  probably 
highly  dispersed;  but,  more  than  this,  it  is  probably  held  so 
fast  or  is  so  surrounded  by  the  gelatinous  hydroxids  and 
silicates  of  aluminium  and  iron  constituting  these  earths 
that  the  ordinary  washing  schemes  do  not  suffice  to  extract 
the  gold.  The  gold  is  " masked"  by  the  hydrated  colloids 
just  as  iron  is  masked  through  the  presence  of  organic  sub- 
stances —  in  other  words,  the  ordinary  analytical  reactions 
of  the  metal  are  not  obtained  until  the  organic  parts  have 
been  destroyed.  In  order  to  make  use  of  these  minerals,  it 

1  See  A.  CAVAZZI,  Koll.-Zeitschr.,  12,  196  (1913). 


196  COLLOID  CHEMISTRY 

would  be  necessary  to  destroy  the  various  inorganic  colloids 
or  to  separate  the  metal  from  its  adsorption  complexes  —  a 
colloid-chemical  problem  which  up  to  the  present  has  not 
been  solved  satisfactorily.  Even  relatively  pure  gold  when 
colloidally  dispersed  is  not  taken  up  easily  when  shaken  with 
mercury  —  a  fact  no  doubt  attributable  to  the  difficulties 
incident  to  obtaining  adequate  contact  between  the  colloid 
particles  and  the  surface  of  the  mercury. 

Another  phenomenon  in  metallurgy,  colloid-chemical  in 
nature,  is  seen  in  the  deposition  of  metals  by  electrolytic 
means.  The  structure  of  an  electrolytically  produced  pre- 
cipitate is  markedly  influenced,  for  instance,  through  the 
addition  of  traces  of  organic  colloids  like  gelatin,  albumin  or 
dextrin.  When  present  in  certain  definite  concentrations, 
these  colloids  bring  about  a  great  increase  in  the  degree  of 
dispersion  of  the  electrolytically  produced  precipitate.  In- 
stead of  voluminous  macro-crystalline  or  micro-crystalline 
precipitates,  there  are  obtained  dense,  finely  structured 
layers  showing  a  smooth  polish.  The  Germans  call  this 
"Glanzgalvanisation."  l  Colloid-chemically  we  may  under- 
stand what  happens  by  remembering  that  the  added  colloids 
are  of  the  group  of  the  hydrated  emulsoids.  As  the  presence 
of  tannin  or  gelatin  tends  to  produce  gold  in  highly  dispersed 
form,  just  so  do  the  hydrated  emulsoids  act  in  the  case  of 
the  electrolytic  deposition  of  the  metals  —  a  process  which 
also  represents  a  condensation  of  highly  dispersed  particles 
to  grosser  ones.  I  can  only  mention  in  passing  that  the 
making  of  mirrors,  like  those  of  silver,  represents  an  analo- 
gous dispersoid-chemical  process.2 

§10. 

The  most  important  applications  of  colloid  and  dispersoid 
chemistry  are,  however,  to  be  found  in  the  metallurgy  of 

1  See,  for  example,  E.  MULLER,  Zeitschr.  f.  Elektrochemie,  317  (1906); 
a  review  is  found  in  Koll.-Zeitschr.,  1,  60  (1906). 

2  See  V.  KOHLSCHUTTER,  Koll.-Zeitschr.,  12,  285  (1912);   further  papers 
are  listed  in  the  indices  of  the  Kolloid-Zeitschrift  from  12  on. 


TECHNICAL  APPLICATIONS  197 

the  alloys  —  more  especially  in  the  metallurgy  of  iron  and 
steel.  Because  of  the  newness  of  the  point  of  view  and  the 
importance  of  the  subject,  I  beg  you  to  let  me  enter  into  a 
few  details. 

As  you  know,  the  alloys,  like  the  different  steels  and  irons, 
show  differences  not  only  in  chemical  composition  but  in 
structure.  You  are  all  familiar,  for  example,  with  the 
coarse  fracture  of  the  ordinary  cast  iron  as  compared  with 
the  microscopic  or  even  sub-microscopic  structure  of  the 
finer  grades  of  steel.  You  also  know  that  even  with  con- 
stancy in  chemical  composition  one  and  the  same  steel  may 
show  very  different  structures  and  that  the  nature  of  these 
structures  is  much  influenced  by  variations  hi  the  tempera- 
tures to  which  the  metal  has  been  exposed,  by  the  suddenness 
with  which  these  temperature  changes  have  been  brought 
about,  by  tempering,  by  mechanical  stresses  and  by  simple 
ageing.  It  is  possible,  hi  other  words,  to  obtain  from  one 
and  the  same  mixture  of  iron  and  carbon  a  series  of  disper- 
soids  showing  different  degrees  of  dispersion  which  may 
range  from  the  type  of  the  coarsely  crystalline  to  that  of  the 
microscopically  non-crystalline  " solid"  solution.  We  dis- 
cover a  series  of  dispersoids  of  iron  and  carbon  (and  the  same 
is  true  of  other  alloys)  entirely  analogous  to  the  series  of 
dispersoids  which  I  showed  you  in  the  case  of  sulphur,  of 
sodium  chlorid,  of  silicic  acid,  etc. 

This  simple  and  entirely  familiar  observation  that  two 
metals  may  be  mixed  into  each  other  with  the  subdivided 
particles  showing  different  degrees  of  dispersion,  becomes 
tremendously  important  in  the  light  of  our  knowledge  of 
the  dispersed  systems.  The  technical  and  physico-chemical 
properties  of  an  alloy  are  largely  dependent  upon  the  size  of 
the  subdivided  particles  constituting  it.  Coarsely  structured 
alloys  are,  as  a  rule,  brittle  and  non-elastic,  while  in  the 
words  of  the  metallurgist,  W.  GURTLER,  "a  finely  granular 
material  and  the  absence  of  every  marked  or  sharply  defined 
structure  are  the  signs  of  a  mechanically  valuable  product." l 

*  See  W.  GURTLER,  Handbuch  der  Metallographie,  1,  II,  450,  Berlin,  1913. 


198  COLLOID  CHEMISTRY 

Even  though  this  relation  between  degree  of  dispersion 
and  technical  properties  has  been  generally  recognized,  it 
has  by  no  means  been  regarded  as  of  the  importance  which 
modern  authors,  among  whom  I  should  like  to  count  myself, 
have  assigned  to  it.  Two  other  physico-chemical  principles 
are  still  assumed  to  be  of  chief  importance,  the  phase  rule 
and  VAN'T  HOFF'S  concept  of  the  solid  solution,1  while  the 
relation  between  size  of  granule  and  physical  properties  has 
been  regarded  as  of  only  secondary  significance. 

Numerous  texts  are,  of  course,  available  which  deal  with 
these  obvious  physical  relationships,  though  their  number 
is  greatly  exceeded  by  such  as  deal  with  the  more  purely 
chemical  aspects  of  the  problem.  What  has  been  lacking 

1  In  spite  of  my  great  admiration  for  the  progress  that  has  been  made  in 
metallography  through  the  introduction  into  this  field  of  the  concepts  of 
chemical  equilibrium,  the  phase  rule  and  VAN'T  HOFF'S  concept  of  solid 
solution,  I  cannot  help  emphasizing  the  need  of  caution  in  all  this,  for  these 
concepts  are  all  based  upon  the  truth  of  certain  assumptions.  The  concept 
of  equilibrium,  for  example,  assumes  that  we  deal  with  states  of  equilibrium 
which  under  experimental  conditions  may  be  reached  from  either  side.  It 
is,  however,  characteristic  of  alloys  (like  the  steels)  that  the  changes  taking 
place  in  them  never  come  to  a  stop.  The  belief  that  true  equilibria  are 
attained  in  these  solid  mixtures  therefore  lacks  support.  It  seems  worthy 
of  note  that  we  find  in  the  metallic  world  no  reference  to  the  fact  that  GIBBS' 
phase  rule  may  be  applied  only  if  the  conditions  presupposed  existent  by 
the  author  of  the  rule  are  really  present  and  since  these  are  not  satisfied  in 
the  majority  of  the  alloys,  application  of  the  rule  is  therefore  forbidden. 
GIBBS'  rule  is  valid  only  for  equilibria  in  systems  in  which,  to  use  the  words 
of  WILLARD  GIBBS  himself,  the  energies  existent  in  the  surfaces  of  the  phases 
composing  the  system  may  be  ignored.  It  is  valid,  in  other  words,  only 
for  macro-heterogeneous  systems.  These  conditions  are  not  fulfilled  in  the 
case  of  the  dispersoid  alloys,  particularly  not  in  those  which  are  technically 
most  important  and  in  which  the  degree  of  dispersion  is  particularly  high. 
GIBBS'  phase  rule  cannot  be  applied  to  these  alloys  any  more  than  it  can  be 
to  ordinary  liquid  colloids.  Metallurgy,  moreover,  deals  almost  exclusively 
with  solid  solutions  in  the  sense  of  VAN'T  HOFF,  hi  other  words,  with  molec- 
ularly  dispersed  solutions.  It  ignores  practically  entirely,  therefore,  the 
question  of  degree  of  dispersion.  The  technically  far  more  important  colloid 
solid  solutions,  to  the  wide  distribution  of  which  I  called  attention  some 
nine  years  ago  [see,  for  example,  the  article  of  P.  P.  VON  WEIMARN,  Koll.- 
Zeitschr.,  7,  35  (1910)],  have  to  the  present  tune  received  no  conscious  treat- 
ment in  metallography  excepting  by  C.  BENEDICKS  (references  to  whose 
work  are  given  later). 


TECHNICAL  APPLICATIONS  199 

has  been  a  proper  emphasis  upon  the  tremendous  importance 
of  size  of  granules  to  physical  properties  and  relative  lack  of 
importance  of  the  specific  chemical  composition  of  the  alloy. 
There  was  missing,  to  put  it  briefly,  a  proper  correlation  of 
the  phenomena  observable  in  these  fields  with  the  analogous 
phenomena  observable  in  other  fields  and  in  which  these 
relationships  could  be  followed  in  simpler  and  more  general 
fashion.  This  correlation  is  splendidly  made  when  it  is 
recognized  that  the  problems  of  the  physical  chemistry  of 
the  alloys  is  that  of  the  physical  chemistry  of  the  dispersed 
systems  and  of  colloid  chemistry.  For  what  is  the  study  of 
the  dispersed  systems  but  that  of  the  relation  between  size  of 
granules  and  physico-chemical  properties,  and  what  is  colloid 
chemistry  but  the  science  of  a  special  subdivision  in  this  realm? 
And  have  we  not  found  that  changes  in  degree  of  dispersion 
constitute  a  factor  which  brings  about  incomparably  more 
radical  variations  in  the  physico-chemical  properties  of  any 
dispersoid  than  are  observed,  for  instance,  between  the 
properties  of  different  steels?  Colloid  chemistry  has  shown 
us  that  the  physico-chemical  properties  of  the  dispersoid 
change  with  changes  in  the  size  of  the  dispersed  granules 
and  that  these  differences  in  degree  of  dispersion  are  the 
chief  factors  of  moment,  distinguishing  the  colloids  in  this 
way  from  the  molecular  solution  on  the  one  hand  and  the 
coarse  suspension  on  the  other,  while  at  the  same  time 
serving  to  unite  the  two.  From  the  teachings  of  colloid 
chemistry,  it  cannot  be  gainsaid  that  in  metallurgy,  too,  the 
relation  between  size  of  granule  and  properties  belongs  to 
the  most  important  and  most  widespread  of  all  the  relation- 
ships with  which  we  are  familiar  in  modern  physical  chem- 
istry.1 

1  W.  GURTLER  in  his  admirable  handbook  lays  more  emphasis  upon  these 
important  relations  between  the  degree  of  dispersion  and  physico-chemical 
properties  than  any  other  author,  though,  to  my  mind,  still  not  enough. 
See  in  this  connection  the  paper  of  P.  P.  VON  WEIMARN,  Internat.  Zeitschr. 
f.  Metallographie,  65  (1911),  as  well  as  the  remarks  of  W.  GURTLER  following 
this  paper. 


200  COLLOID  CHEMISTRY 

111- 

If  we  try  to  classify  according  to  degree  of  dispersion  the 
different  structural  elements  which  enter  into  the  iron- 
carbon  compounds,  we  may  begin  with  the  three  metarals, 
iron  or  ferrite,  with  which  may  be  included  its  various  allo- 
tropic  forms,  iron  carbid  (cementite)  and  carbon.  Carbon 
appears  in  the  dispersoid  series:  graphite,  tempering  carbon, 
hardening  carbon. 

Graphite  is  a  coarsely  dispersed  carbon;  hardening 
carbon,  an  extraordinarily  highly  dispersed  one.  Accord- 
ing to  prevalent  hypothesis,  the  carbon  in  this  material  is 
supposed  to  exist  in  molecular,  solid  solution.  Tempering 
carbon  occupies  a  middle  position  in  the  matter  of  degree  of 
dispersion  and  physico-chemical  properties.  The  allotropy 
of  these  carbons  is  a  dispersion  allotropy.  The  degree  of 
dispersion  in  hardening  carbon  is,  perhaps,  not  entirely 
molecular.  It  apparently  occupies  a  transition  point  be- 
tween colloidally  and  molecularly  dispersed  solid  solutions. 
According  to  some  of  my  still  unpublished  experiments,  the 
so-called  EGGERTZ  solutions  of  this  hardening  carbon,  as 
obtained  by  solution  of  steel  in  dilute  nitric  acid,  are  obvi- 
ously colloid  in  nature.  They  scarcely  dialyze,  are  ultra- 
microscopically  heterogeneous,  etc. 

Iron  carbid  or  cementite  also  presents  different  degrees  of 
dispersion.  Primary  cementite  is  coarsely  crystalline,  seg- 
regated cementite  somewhat  finer,  perlitic  cementite  finer 
still.  The  finest  of  this  series  of  materials  may  show  so 
slight  a  structure  that,  in  the  words  of  W.  GURTLER,  it 
represents  "an  almost  molecularly  dispersed  solid  solution." 

Even  ferrite  presents  different  degrees  of  dispersion, 
though  the  relationships  here  are  complicated  by  the  fact 
that  iron  by  itself  appears  in  a  number  of  different  forms 
which  are  allotropic  in  the  ordinary  sense  of  the  word. 


TECHNICAL  APPLICATIONS  201 

§12. 

After  these  primary  elements  we  need  to  consider  the 
innumerable  secondary  ones  which  result  from  their  com- 
bination. Of  especial  importance  are  mixtures  of  ferrite 
and  carbon  and  of  ferrite  and  cementite.  From  the  many 
illustrations  available,  I  touch  upon  a  group  which  is 
interesting  because  of  its  importance  in  the  technology  of 
steel,  and  because  it  concerns  a  field  which  has  been  analyzed 
dispersoid-chemically  by  the  Swedish  investigator,  C.  BENE- 
DICKS. 1  I  refer  to  the  series  obtained  when  steel  is  chilled 
at  different  rates,  namely,  austenite,  martensite,  troostite, 
osmondite,  sorbite  and  perlite.  In  this  series  austenite  and 
martensite  (perhaps,  also,  hardenite)  are  the  most  highly 
dispersed.  They  are  probably  molecularly  dispersed  mix- 
tures obtained  when  the  chilling  is  brought  about  very 
rapidly.  Perlite  is  obtained  with  slower  cooling.  It  shows2 
characteristic  lamella-like  deposits  of  cementite  which  are 
easily  visible  microscopically  and  sometimes  even  macro- 
scopically.  Perlite  is,  in  other  words,  a  relatively  coarse 
dispersoid.  Between  these  extremes  of  martensite  and 
perlite  are  found  those  which  bridge  the  gap  in  perfectly 
smooth  fashion,  namely,  troostite,  osmondite  and  sorbite. 
It  is  for  this  reason  that  BENEDICKS  came  to  the  conclusion 
that  these  intermediates  represent  solid  colloid  solutions. 

1  C.  BENEDICKS,  Zeitschr.  f.  physikal.  Chem.,  62,  6  (1905);    Jour.  Iron 
and  Steel  Inst.,  352  (1905);   Roll. -Zeitschr.,  7,  290  (1910).     In  studying  the 
well-known  text  on  siderology  of  H.  VON  JUPTNER,  without  being  aware  of 
the  contributions  of  BENEDICKS,  I  arrived,  in  1909,  at  a  colloid-chemical 
view  of  metallographic  processes,  the  theoretical  and  experimental  results  of 
which  were  published,  in  part,  in  some  of  the  papers  which  I  have  cited. 
Not  until  later,  after  I  had  made  a  whole  series  of  colloid-chemical  experi- 
ments, as  with  the  EGGERTZ'  solutions,  did  I  run  across  the  short  first  paper 
of  C.  BENEDICKS,  cited  above,  in  which  he  expressed  in  very  clear  fashion 
a  part  of  these  colloid-chemical  notions.     It  was  after  this  that  I  wrote  to 
C.  BENEDICKS  asking  him  to  collect  his  views  and  to  allow  me  to  publish 
them  in  the  Kolloid-Zeitschrift. 

2  Beautiful  illustrations  of  perlite  structure  may  be  found  in  the  article 
of  C.  BENEDICKS,  Roll. -Zeitschr.,  7,  290  (1910).     The  structure  reminds  one 
strongly  of  LIESEGANG'S  rings. 


202  COLLOID   CHEMISTRY 

Troostite  is  evidently  a  colloid  solution  of  cementite  in  ferrite; 
perlite  is  the  coarsely  dispersed  or  coagulated  product  of  this 
cementite-ferrite  sol. 

Faith  in  the  existence  of  these  colloid  intermediates  seems 
not  only  justified  but  absolutely  necessary  when  it  is  re- 
membered that  molecularly  dispersed  or  coarsely  dispersed 
metarals  in  moving  from  the  one  class  into  the  other  must  pass 
through  the  colloid  realm.  The  only  question  at  stake  is 
whether  it  is  possible  to  fix  the  material  at  the  moment  in 
which  it  is  passing  through  this  middle  region. 

But  to  this  end  solid  dispersion  media  and  the  sensitive- 
ness of  alloys  to  external  conditions  obviously  offer  the 
best  possible  opportunities.  Troostite  is  best  produced 
through  careful  reheating  of  steels.  This  is  no  doubt  be- 
cause hi  this  fashion  a  condensation  is  brought  about  of  the 
previously  molecularly  dissolved  carbon  or  cementite  parti- 
cles contained  in  the  original  martensite  or  austenite.  The 
heating  process  and  its  results  are,  in  other  words,  analogous 
to  the  repeatedly  discussed  conditions  best  designed  to  yield 
colloid  solutions  of  gold,  of  sodium  or  of  sulphur  from  their 
molecularly  dispersed  solid  solutions.  As  known  to  every- 
one, carefully  regulated  increases  and  decreases  in  tempera- 
ture are  constantly  used  in  the  manufacture  of  metallic 
products  to  get  these  to  show  as  fine  a  grain  as  possible. 

But  of  especial  importance  are  the  relations  which  exist 
between  these  differently  dispersed  states1  of  the  metarals 
and  the  technical  and  physico-chemical  properties  of  the 
resulting  metallic  products.  As  emphasized  by  C.  BENE- 
DICKS, the  efforts  of  the  metallurgist  are  constantly  directed 
toward  the  obtaining  of  a  maximal  elasticity  and  toughness 
of  his  technically  important  alloys.  To  accomplish  this  he 

1  W.  GURTLER  (I.e.)  has  also  repeatedly  emphasized  that  not  a  definite 
chemical  or  physical  property,  but  rather  a  definite  structural  state  —  in 
other  words,  a  definite  relationship  of  the  differently  dispersed  metarals 
to  each  other  —  is  characteristic  of  the  technically  important  iron  alloys, 
and  that  many  of  the  names  given  to  structural  elements  do  not,  as  a  matter 
of  fact,  refer  to  different  individual  structural  elements,  but  cover  their  state 
of  subdivision  and  of  admixture  with  each  other. 


TECHNICAL  APPLICATIONS  203 

tries,  as  far  as  possible,  to  get  all  the  constituents  of  his 
alloys  into  a  colloid  degree  of  dispersion.  As  BENEDICKS 
puts  it,  "a  correctly  produced  hair  spring  consists  of  troo- 
stite;  the  cry  is  always  for  steel  rails  consisting  of  sorbite; 
the  tendons  of  technology,  our  steel  cables,  are,  like  the 
tendons  of  the  human  body,  colloid  in  structure.  The 
dispersion  medium  in  all  these  instances  is  a  distinctly 
crystalline  body,  the  separate  crystals  of  which  man  has  for 
ages  past  tried  to  keep  just  as  small  as  possible." 

What  is  interesting  in  conjunction  with  these  observations 
is  that  many  of  the  most  important  technological  properties 
of  the  metallic  alloys  attain  their  optimum  in  a  region  of 
medium  or  colloid  degree  of  dispersion  and  not,  for  example, 
in  a  higher  or  molecular  one.  The  property  of  hardness, 
for  instance,  grows  steadily  with  increase  in  degree  of  dis- 
persion, yet  a  suddenly  cooled,  "hard  as  glass,"  austenitic 
steel  becomes  so  brittle  that  the  progressive  increase  in 
hardness  soon  has  a  limit  put  upon  it.  Elasticity,  tough- 
ness, modulus  of  rupture,  rate  of  solution  in  dilute  acids, 
coloration  intensity  of  structure  through  iodin  or  picric  acid, 
all  these  and  many  other  properties  attain  their  optimum  in 
a  region  of  medium  grade  of  dispersion.  Such  facts  regard- 
ing the  alloys  parallel  the  variations  in  color,  opacity, 
viscosity,  etc.,  which  we  discussed  in  our  second  lecture, 
where  we  also  discovered  that  a  medium  or  colloid  degree 
of  dispersion  allowed  these  properties  to  appear  in  most 
intense  form.  The  curves  expressive  of  the  solution  velocity 
of  a  series  of  alloys  (martensite,  osmondite,  troostite,  per- 
lite),  in  which  a  maximum  is  observed  in  the  middle,  the 
curves  which  show  that  in  cast  iron  there  exists  an  optimum 
for  the  degree  of  dispersion  of  the  graphite  and  that  this 
corresponds  with  a  maximum  of  carrying  strain,1  these  are 
curves  thoroughly  familiar  to  every  colloid  chemist. 

But  other  metallurgical  phenomena  find  a  parallel  hi 
colloid-chemical  ones.  Certain  steels  on  cooling  show  a 

1  See  W.  GURTLER,  Handbuch,  I.e.,  1,  II,  308  (experiments  of  HEYN  und 
LEYDE). 


204  COLLOID  CHEMISTRY 

viscosity  maximum  somewhere  below  1700°  C.  With  fall- 
ing temperature  they  do  not  become  progressively  more 
viscid  as  do  normal  liquids  but  exhibit  a  great  increase  in 
viscosity  followed  by  a  sudden  decrease.1  This  is  the  same 
behavior  which  we  encountered  previously  in  the  separa- 
tion phenomena  of  hydrated  emulsoids,  of  critical  fluid 
mixtures,  of  crystalline  liquids  and  of  molten  sulphur.  The 
parallelism  justifies  the  conclusion  that  in  the  case  of  the 
molten  steels  there  also  occurs  at  the  temperatures  under 
discussion  a  separation  in  highly  dispersed  or  colloid  form 
which  with  still  greater  lowering  of  temperature  yields  a 
coarsely  dispersed  system.  It  is  possible  that  the  so-called 
pseudoeuctectoid  melts2  are  like  these  liquid  steels.  Accord- 
ing to  R.  LORENZ  it  is  possible  to  make  colloid  solutions  in 
molten  salts,  the  so-called  "pyrosols." 

A  further  analogy  of  metallurgical  behavior  to  that  of  the 
colloids  is  seen  in  the  fact  that  the  melting  point  of  ordinary 
grey  iron,  for  example,  is  decidedly  higher  than  its  solidifica- 
tion point.3  The  same  phenomenon  may  be  observed  in 
any  gelatin  gel.  The  explanation  hi  both  cases  seems  to  be 
that  during  and  after  solidification,  the  particles  grow  to 
larger  aggregates;  and  the  melting  point  of  large  particles 
is  decidedly  higher  than  that  of  more  highly  dispersed  ones. 

It  must  also  be  pointed  out  that  when  the  carbon  content 
of  a  steel  reaches  a  concentration  of  about  0.45  percent,  a 
peculiar  change  in  structure4  takes  place.  While  a  so-called 
steel  structure  is  characteristic  of  the  higher  carbon  contents, 
there  appears  a  so-called  granular  structure  when  the  con- 
centration of  carbon  is  less.  An  analogous  difference  in 
structure  characterizes,  for  instance,  the  setting  of  a  con- 
centrated and  a  dilute  gelatin  if  this  contains  a  little  alcohol.5 

I  beg  to  conclude  these  remarks  on  the  relation  of  colloid 

See  W.  GURTLER,  I.e.,  131. 
See  W.  GURTLER,  I.e.,  188. 
See  W.  GURTLER,  I.e.,  186. 
See  W.  GURTLER,  I.e.,  384. 

See,  for  example,  WOLFGANG  OSTWALD,  Grundriss  der  Kolloidchemie, 
1.  Aufl.,  350,  Dresden,  1909. 


TECHNICAL  APPLICATIONS  205 

chemistry  to  metallurgy  by  pointing  out  that  the  phenomena 
of  ageing,  of  fatigue  and  of  distortion  in  alloys,  whatever 
their  kind,  all  tend  in  general  toward  a  decrease  in  degree  of 
dispersion  —  a  behavior  analogous,  therefore,  to  that  dis- 
cussed previously  as  characteristic  of  these  same  phenomena 
in  colloid  systems.  It  is  as  true  of  a  steel  as  of  a  colloid  that 
the  slightest  alterations  in  external  or  internal  conditions 
bring  about  great  changes  in  its  state.  Things  are  no  more 
at  rest  in  a  steel  than  in  any  colloid  mixture. 

§13. 

This  brings  us  to  the  important  and  varied  applications 
which  have  been  made  of  colloid  chemistry  to  the  organic 
industries  and  the  technical  arts.  We  are  justified,  as  a 
matter  of  fact,  in  designating  these  as  the  colloid  industries 
or  the  colloid-chemical  arts. 

To  begin  with  the  latter  heading,  we  may  take  up,  as  of 
first  importance,  the  processes  of  dyeing  and  tanning.1  Of 
the  many  available  illustrations  I  must  again  choose  an 
arbitrary  few.  In  the  first  place,  we  must  not  in  these 
fields  make  the  mistake  of  considering  everything  we  see  as 
exclusively  colloid-chemical  in  nature.  Dyeing  and  tanning 
are  complex  procedures  which  ami  at  definite  end  results 
but  they  make  no  assumptions  regarding  the  nature  of  the 
processes  which  are  to  lead  to  such  results.  A  dyeing  or 
tanning  effect  may  be  accomplished  by  very  different 
methods,  in  the  list  of  which  appear  many  non-colloid- 
chemical  ones.  Some  colloid-chemical  processes  must,  of 
course,  always  appear  as  long  as  the  material  to  be  dyed  or 
to  be  tanned  is  itself  colloid  in  nature.  In  practice  this  is 
nearly  always  the  case,  for  not  only  are  textiles  and  hides 

1  For  a  discussion  of  dyeing  from  a  colloid-chemical  point  of  view,  see 
J.  PELET-JOLIVET,  Die  Theorie  des  Farbeprozesses,  Dresden,  1912,  as  well 
as  his  numerous  papers  in  the  Kolloid-Zeitschrift.  For  the  colloid  chemistry 
of  tanning,  see  the  reviews  of  E.  STIASNY,  Koll.-Zeitschr.,  2,  257  (1908)  and 
CHR.  NEUNER,  ibid.,  8,  329  (1910);  9,  65,  144  (1911).  Numerous  other 
papers  upon  these  subjects  by  other  authors  may  be  found  in  the  Kolloid- 
Zeitschrift  and  in  the  Kolloidchemische  Beihefte. 


206  COLLOID  CHEMISTRY 

typical  gels  or  mixtures  of  such,  but  just  as  every  chemical 
reaction  into  which  a  colloid  component  enters  must  in 
consequence  show  certain  colloid-chemical  peculiarities,  just 
so  must  even  the  so-called  " purely"  chemical  dyeing  and 
tanning  processes  —  as  dyeing  with  an  oxidizing  agent  or 
tanning  with  formaldehyde  —  show  colloid-chemical  pecu- 
liarities.    But  in  many  cases  there  is  added  to  this  colloid- 
chemical  component  resident  in  the  substance  itself  that 
second  one  due  to  the  fact  that  the  dye  bath  or  the  tanning 
solution  is  colloid  in  nature.     Many  organic  dyes  are  colloid 
when  dissolved  in  water,  and  the  same  is  true  of  nearly  all 
the  vegetable  tanning  materials,  such  as  tannin  and  the 
different  bark  extracts.     Many  mineral  substances  as  used 
in  tanning  are  also  colloid,  as  chromium  hydroxid,  sulphur, 
etc.     In  all  these  instances  we  therefore  deal  with  reactions 
between  at  least  two  colloids,  and  to  these  is  often  added  the 
effect  of   several  more  colloids.     Thus   in  certain   dyeing 
processes  organic  colloids  like  tannin  or  inorganic  colloids 
like  aluminium  hydroxid  are  often  added.     In  all  such  col- 
loid reactions  purely  chemical  ones  come  to  play  a  decidedly 
minor  role  when  compared  with  the  adsorption  effects  and 
all  their  possible  secondary  reactions  which  we  discussed  in 
a  previous  lecture.     As  in  the  simpler  examples  of  adsorp- 
tion, we  need  in  the  adsorption  of  dyes  and  of  tanning 
materials  to  recognize  a  whole  series  of  different  reactions  as 
taking  place  side  by  side.     In  these  there  may  predominate 
at  one  time  electrical  effects  due  to  electrical  differences 
between  substrate  and  materials  to  be  adsorbed,  at  another 
time  surface  tension  effects,  at  a  third,  chemical  ones.     The 
literature    abounds,    therefore,    in    electrical,    mechanical, 
chemical   and   other  theories   of  dyeing  and   of  tanning. 
These  all  tend  to  err  in  that  they  incline  to  place  some  one 
of  these  theories  over  and  against  some  other;   but  as  we 
found  in  our  discussion  of  adsorption,  the  changes  char- 
acteristic of  this  may  be  brought  about  in  several  totally 
different  fashions,  in  which  any  one  may  be  quite  as  impor- 
tant as  any  other. 


TECHNICAL  APPLICATIONS  207 

Of  great  importance  in  dyeing  is  not  only  the  accumula- 
tion of  the  dye  in  the  material  to  be  dyed,  but  its  fixation 
in  the  material.  The  dye  must  be  united  in  irreversible 
fashion  to  the  fiber.  The  mistake  is  often  made  of  alleging 
that  adsorption  alone  cannot  explain  "fast"  dyeing  since, 
by  definition,  adsorption  is  always  a  reversible  process. 
This  view  forgets  that  it  all  depends  upon  the  intensity  of 
the  adsorption  whether  on  rinsing  in  the  pure  dispersion 
medium  some  of  the  adsorbed  material  will  again  go  back 
into  solution  or  not.  If  the  adsorption  is  so  intense  that 
practically  all  of  the  dye  is  taken  out  of  the  dye  bath  —  and 
this  is  obviously  the  most  economical  method  of  dyeing  — • 
then  none  of  the  adsorbed  dye  is  likely  to  pass  out  of  the 
dyed  material  into  the  pure  water,  for  this  represents  a  still 
more  dilute  solution  of  the  dye  than  did  the  almost  de- 
colorized dye  bath  which  was  in  equilibrium  with  the  con- 
centration of  the  dye  in  the  dyed  textile.  While  adsorption 
alone  may  therefore  lead  to  "fast"  dyeing,  secondary 
changes  like  polymerizations,  secondary  decompositions, 
direct  chemical  unions  between  fiber  and  dye,  etc.,  often 
come  or  are  brought  about  to  yield  the  ultimate  "fast" 
result.1 

In  similar  fashion  the  adsorption  of  tanning  materials  by 
a  hide  is  also  followed  by  secondary  and  presumably  chemical 
processes,  more  particularly  changesrin  the  physical  state 
of  the  hide  such  as  dehydration  and  coagulation.2  It  is 
through  these  changes  that  the  product  obtains  its  techni- 
cally valuable  properties.  Let  me  emphasize,  in  passing, 
that  in  both  the  process  of  dyeing  and  of  tanning,  the  gel 
substrate  is  prepared  beforehand  for  a  better  adsorption  by 
being  treated  with  acids  or  alkalies  which  tend  to  hydrate 
and  to  increase  the  degree  of  dispersion  of  the  materials 
making  up  the  substrate.  Subsequently,  through  the  addi- 
tion of  salts,  an  attempt  is  made  to  coagulate  the  colloid 
dyes  or  tanning  materials  in  the  fibers. 

1  See  the  main  text  for  a  discussion  of  secondary  reactions  consequent 
upon  adsorption. 

2  See  especially  J.  VON  SCHROEDER,  Kolloidchem.  Beih.,  1,  1  (1908). 


208  COLLOID  CHEMISTRY 

§14. 

Industries  which  may  in  the  true  sense  of  the  word  be 
called  colloid  industries  are  seen  in  the  group  of  those  which 
work  with  cellulose.  Pure  cellulose  is  already  a  typical  gel 
possessed  of  a  beautiful  ultramicroscopic  structure ;  it  shows 
typical  swelling  phenomena  and  on  solution  yields  the  highly 
viscid  liquids  which  are  characteristic  of  the  hydrated 
emulsoids.  It  may  be  precipitated  from  these  solutions 
through  neutral  salts  or  through  dehydrating  agents  like 
alcohol. 

These  phenomena  are  all  reversible.  Of  especial  interest 
is  a  series  which  is  irreversible  in  nature.  I  refer  to  the 
processes  of  parchment  manufacture  and  of  mercerization. 
Cellulose  (in  the  form  of  filter  paper  or  cotton,  for  example) 
swells  in  acids  and  alkalies  of  medium  concentration  more 
than  in  water,  just  as  does  gelatin  or  fibrin  under  similar 
circumstances.  But  following  this  treatment  there  occur 
various  secondary  changes  which  upon  removal  of  the 
alkali  and  drying  of  the  gel  leave  this  in  a  much  more  highly 
dispersed  and  voluminous  state  than  before.  We  have 
before  us,  then,  parchment  paper  or  mercerized  cotton, 
either  of  which  now  shows  new  optical  and  mechanical 
properties  to  correspond  with  the  colloid-chemical  changes 
in  state  that  it  has  suffered. 

Various  derivatives  *of  cellulose  and  various  combinations 
of  cellulose  with  other  materials  in  the  form  of  solid  solutions 
show  typical  colloid-chemical  behavior.  Best  known  are 
the  cellulose  gels  used  for  the  production  of  artificial  silks 
and  various  plastic  masses.  Alkali-cellulose,  like  alkali 
gelatin,  when  treated  with  carbon  bisulphid  yields  a  remark- 
able substance  known  as  viscose.  From  viscose  is  today 
prepared  most  of  the  artificial  silk  —  an  industry  which 
already  has  an  annual  value  of  $200,000,000.  Viscose 
shows  all  the  characteristics  of  a  hydrated  emulsoid.  The 
whole  process  of  viscose  silk  manufacture  is  naturally  honey- 
combed with  colloid-chemical  phenomena.  The  freshly 
prepared  viscose  is  first  aged  or  ripened  before  it  yields  a 


TECHNICAL  APPLICATIONS  209 

product  optimal  for  spinning.  The  velocity  of  this  internal 
change  in  state  can  be  influenced  through  the  addition  of 
various  substances.  The  originally  fluid  fibers  must  be 
coagulated;  to  accomplish  this  end  the  patent  literature  is 
filled  with  such  a  long  series  of  different  processes  that  it  is 
safe  to  say  that  this  coagulation  phenomenon  in  viscose 
probably  represents  the  best  studied  of  the  whole  line  of 
coagulations  in  emulsion  colloids.  These  internal  changes 
in  state  must,  moreover,  be  inhibited  at  a  certain  time  else 
the  fiber  becomes  brittle.  The  tendency  of  the  fiber  to 
swell  in  water  or  steam  and  the  consequent  weakening  of 
the  fiber  must  be  reduced  to  a  minimum.  The  whole  in- 
dustry represents  an  unbroken  succession  of  colloid-chemical 
processes. 

Other  cellulose  compounds  and  solutions  of  them  like  that 
of  cellulose  in  ammoniated  copper  oxid  may  be  employed  in 
similar  fashion.  Artificial  silk  can  also  be  prepared  from 
gelatin,  though  when  this  is  used  other  methods  of  coagula- 
tion must  be  employed  than  in  the  case  of  viscose.  Cellu- 
lose esters  may  be  used  to  prepare  transparent  varnishes 
such  as  are  employed  upon  aeroplanes,  and  these  same 
materials  are  used  for  the  production  of  plastic  masses  of 
various  types.1  The  best  known  of  these  plastic  masses  is 
celluloid,  that  peculiar  and  scientifically  little  studied  solid 
solution  of  cellulose  and  camphor,  the  uses  of  which  are 
familiar  to  every  one.  Because  of  its  high  inflammability 
numerous  cellulose  derivatives  such  as  cellon  and  acetyl 
cellulose  ester  have  been  introduced  to  take  its  place. 

I  should  like  to  call  your  attention  to  some  further  plastic 
masses  which  are  also  colloid  gels.  Galalite  is  nothing  but 
a  casein,  so  treated  that  it  no  longer  shows  any  powers  of 
swelling.  Bakelite  is  a  condensation  product  derived  from 
phenol  in  the  presence  of  alkali  and  formaldehyde.  It  may 
be  obtained  in  all  states  of  aggregation  varying  from  a  soft, 

1  Regarding  the  manufacture  of  plastic  masses  and  of  artificial  textiles  — • 
especially  such  as  are  derived  from  cellulose  —  see  the  review  of  J.  G.  BELTZER, 
Koll.-Zeitschr.,  8,  177,  313  (1911). 


210  COLLOID  CHEMISTRY 

jelly-like  material  to  a  brittle,  almost  stony,  resin-like 
material.  Bakelite  is  interesting  colloid-chemically  because 
it  is  a  typical  isocolloid,  that  is  to  say,  a  dispersoid  in  which 
the  dispersed  phase  and  the  dispersion  medium  are  polymers 
of  each  other.  We  shall  shortly  come  upon  another  illus- 
tration of  this  group  of  materials. 

§15. 

The  manufacture  of  rubber1  represents  another  typical 
colloid  industry.  A  number  of  colloid  and  dispersoid- 
chemical  processes  are  observable  even  in  the  first  prepara- 
tion of  crude  rubber.  Soft  rubber  comes  from  latex,  which 
consists  of  a  dispersion  of  tiny  droplets  of  tenacious  liquid 
hi  an  albuminous  serum.  Just  as  hi  ordinary  milk,  it  is 
probable  that  the  protein  surrounds  the  soft  rubber  globules 
as  an  adsorption  membrane;  in  other  words,  as  a  so-called 
"haptogenic  membrane."  This  protein  seems  to  play  a 
great  role  not  only  in  the  original  coagulation  of  the  latex, 
but  also  in  giving  to  soft  rubber  its  characteristic  mechanical 
properties.  The  protein  is  apparently  of  the  group  of  the 
globulins,  for  latex  is  commonly  coagulated  through  the 
addition  of  carbonic  acid  or  of  distilled  water,  both  of  them 
typical  coagulation  procedures  for  the  group  of  the  globu- 
lins. In  fact,  all  the  various  coagulation  methods  employed 
seem  to  be  nothing  but  protein  precipitation  methods. 
Their  proper  employment  seems  to  be  a  fundamentally 
important  feature  in  the  whole  process  of  soft  rubber 
preparation.2 

Freshly  prepared  raw  rubber  shows  a  marked  syneresis. 
It  squeezes  off  a  protein-rich  serum.  A  proper  separation 
of  this  material  is  of  great  importance,  since  it  reduces  the 
possibilities  for  bacterial  growth. 

Raw   rubber   swells   tremendously   in   different   organic 

1  Nearly  all  the  newer  papers  covering  the  manufacture  of  rubber  from 
a  colloid-chemical  point  of  view  may  be  found  in  the  original  in  the  Kolloid- 
Zeitschrift  under  the  names  of  WOLFGANG  OSTWALD,  F.  W.  HINRICHSEN, 
B.  BYSOW,  D.  SPENCE,  D.  DITMAR,  P.  SCHIDROWITZ,  etc. 

8  See  also  in  this  connection  Koll.-Zeitschr.,  13,  324  (1913). 


TECHNICAL  APPLICATIONS  211 

solvents,  as  I  have  already  shown  you.  Upon  the  appli- 
cation of  heat,  and  through  mechanical  agitation,  a  large 
part  of  the  soft  rubber  may  be  made  to  go  into  " solution." 
In  this  process  the  remnants  of  coagulated  protein  left  in 
the  soft  rubber  undoubtedly  play  an  important  part,  since 
they  serve  to  counteract  the  tendency  of  the  swollen  rubber 
particles  to  go  into  solution.  These  solutions  of  soft  rub- 
ber behave  like  typical  solvated  emulsoids.  They  show 
great  absolute  viscosities,  great  relative  increases  with  rise 
in  the  concentration  of  the  colloid,  phenomena  of  ageing, 
etc.  All  the  observed  phenomena  are  complicated  through 
the  presence  of  traces  of  the  protein.  Through  the  pres- 
ence of  such  may  apparently  be  explained  the  fact l  that 
upon  addition  of  acids  there  occurs  a  rapid  decrease  in  the 
viscosity  of  soft  rubber  sols.  This  viscosity  of  the  soft 
rubber  is  of  great  importance  in  determining  the  life  of 
the  solid  rubber.  It  has  been  found  that,  as  a  rule,  a 
relatively  high  viscosity  yields  a  lively  rubber.  An  exact 
parallelism  between  the  two  does  not,  of  course,  exist,  nor 
is  this  to  be  expected  as  long  as  we  are  unable  to  control 
the  excessive  sensitiveness  of  such  colloids  to  every  slight 
change  in  their  surroundings. 

Of  great  importance  is  the  process  of  vulcanization  - 
that  series  of  important  physico-chemical  changes  which 
occurs  when  soft  rubber  is  heated  with  sulphur  or  sulphur 
compounds.  At  least  three  different  types  of  changes  are 
to  be  distinguished  from  each  other  in  the  process  of  vul- 
canization: first,  the  taking  up  of  the  sulphur  or  sulphur 
compounds;  second,  their  fixation;  third,  the  changes  in 
state  in  the  rubber  following  therefrom. 

The  nature  of  the  first  of  these  processes  is  still  a  matter 
of  lively  debate.  Some  maintain  it  to  be  a  typical  ad- 
sorption phenomenon,  while  this  view  is  cast  aside  entirely 
by  others.  How  greatly  opinions  differ  is  well  illustrated 
in  two  papers  which  appeared  almost  simultaneously.  In 
one  of  them  the  author  took  it  almost  for  granted  that  a 

i  D.  SPENCE,  Koll.-Zeitschr.,  14,  Heft  6  (1914). 


212  COLLOID  CHEMISTRY 

colloid  like  rubber  should  show  adsorption  phenomena; 
in  the  other,  the  author  concluded  that  not  a  single  fact 
argued  for  the  importance  of  adsorption  in  the  process  of 
vulcanization.  Since  I  was  the  first  to  defend  the  ad- 
sorption notion  —  though  I  did  not  by  any  means  hold 
it  to  be  an  entirely  "  self-evident "  one  —  I  naturally  in- 
cline to  the  view  that  the  taking  up  of  the  sulphur  is  really 
an  adsorption  process.  This  view  is  supported  not  only 
by  the  fact  that  the  process  is  governed  by  the  adsorption 
law,  but  by  the  results  of  D.  SPENCE'S  extraction  experi- 
ments which  show  that  the  amount  of  sulphur  adsorbed 
is  inversely  proportional  to  the  reciprocal  function  of  the 
free  sulphur.1  After  the  sulphur  has  been  taken  up  by 
adsorption,  there  probably  occur  not  only  secondary  chem- 
ical reactions  but  also  changes  in  the  state  of  the  rubber, 
the  end  result  of  all  of  which  is  to  be  interpreted  as  a  de- 
crease in  its  degree  of  dispersion  —  either  a  polymerization 
or  a  coagulation. 

In  passing,  it  is  of  interest  to  point  out  that  radiant 
energy,  as  ultra-violet  light,2  has  been  employed  to  polym- 
erize rubber.  This  is  analogous  to  the  use  of  light  to  bring 
about  coagulation  in  other  colloids,  such  as  the  proteins. 

Permit  me  to  add  a  few  words  regarding  "synthetic" 
rubber,  this  newest  of  the  children  of  organic  chemical 
industry.  This  material,  as  you  know,  is  made  by  polym- 
erizing isopren  and  related  hydrocarbons.  In  the  process 
of  polymerization  the  isopren  passes  through  the  several 
states  of  a  slightly  viscid  liquid  to  a  stiffer  one  to  end  in 
a  hard,  crumbly  mass.  It  is  between  the  two  extremes 
that  we  obtain  a  product  most  like  natural  rubber.  This 
product  may  be  separated  as  a  gelatinous  mass  from  the 
monomeric  mother  substance  through  the  addition  of 

1  In  spite  of  these  corroborative  findings  by  D.  SPENCE  himself,  he  still 
maintains  in  his  recent  papers  that  adsorption  processes  play  no  important 
role  in  the  process  of  vulcanization.     Some  misunderstandings  have  served 
to  complicate  the  discussion.     A  criticism  of  SPENCE'S  most  recently  ex- 
pressed views  is  left  for  the  future. 

2  See  G.  BERNSTEIN,  Koll.-Zeitschr.,  11,  185  (1912);   12,  193,  273  (1913). 


TECHNICAL  APPLICATIONS  213 

alcohol.  The  synthetic  rubber  obtained  by  these  methods 
may  be  used  as  a  class-room  example  of  an  isocolloid  or 
isodispersoid.  In  the  first  stages  of  polymerization  the 
dispersion  medium  consists  of  the  monomeric  mother  sub- 
stance in  which  float  the  polymerized  particles;  in  the 
medium  and  higher  concentrations  there  seems  to  occur 
a  transformation  which  results  in  the  monomeric  compo- 
nent becoming  the  internal  phase  while  the  polymeric 
form  surrounds  it  as  a  gelatinous  structure. 

The  synthetic  product  does  not,  for  the  most  part, 
possess  the  life  or  vulcanization  possibilities  of  natural 
rubber.  The  attempt  has  therefore  been  made  to  over- 
come this  defect  by  introducing  protein  and  similar  col- 
loids into  the  artificial  substance.  Interest  in  synthetic 
rubber  has  recently  subsided  somewhat  because  of  the 
great  fall  in  price  of  the  natural  product.  This  defection 
is,  I  think,  not  merited.  It  seems  to  me  that  interest  in 
artificial  rubber  does  not  today  center  in  the  possibility 
of  finding  a  substitute  for  the  natural  product,  but  more 
in  the  possibility  of  making  a  decidedly  better  one.  The 
situation  is  not  unlike  that  obtaining  in  the  case  of  the 
metallic  alloys.  We  did  not  in  this  field  rest  content  with 
the  utilization  merely  of  such  metallic  mixtures  as  nature 
furnished  us.  Only  as  we  have  treated  these  in  various 
fashions  or  have  added  other  materials  to  them  have  we 
obtained  the  " noble"  alloys  which  now  serve  us.  It 
seems  to  me  as  though  what  we  lack  in  the  manufacture 
of  a  satisfactory  synthetic  rubber  is  the  presence  in  it  of 
a  proper  secondary  colloid  like  protein,  just  as  we  need 
carbon  present  in  iron  to  give  us  steel;  in  fact  this  analogy 
between  rubber  and  steel  seems  to  me  to  be  far  more  than 
a  merely  superficial  one.  By  the  synthesis  of  a  "  noble 
rubber"  materials  might  be  produced  which  would  not 
only  show  properties  superior  to  the  ordinary  natural 
article,  but  of  a  marketable  character. 


214  COLLOID  CHEMISTRY 

§16. 

The  manufacture  of  soap  l  is  another  colloid  industry; 
Through  their  viscosity,  their  powers  of  gelation  and  of 
swelling,  the  salts  of  the  fatty  acids  again  show  themselves 
to  be  typical  emulsoids.  Of  particular  interest  to  the 
colloid  chemist  are  the  technical  processes  involved  in  the 
salting  out  of  the  soaps.  Upon  the  addition  of  sodium 
chlorid,  for  example,  to  the  boiled  soap  solution  there 
follows  a  separation  into  two  liquid  layers,  one  containing 
much  soap  and  little  water,  the  other  (besides  glycerin) 
much  water  and  little  soap.  This  coagulation  phenomenon 
as  carried  out  in  practice  is  one  of  the  most  typical  and 
probably  the  most  gigantic  example  that  nature  offers  of 
the  precipitation  of  a  hydrated  emulsoid.  The  phenom- 
enon proves  on  a  huge  scale  that  in  all  emulsoids  two 
liquid  phases  are  really  divided  into  each  other. 

I  can  only  mention  in  passing  that  hi  the  process  of 
washing,  colloid-chemical  processes  of  all  kinds,  more  par- 
ticularly adsorption  phenomena,  play  a  great  role.2 

§17. 

The  manufacture  of  starch,  of  glue,  of  mucilage  and  of 
fillers  of  various  kinds  is  punctuated  with  colloid-chemical 
processes.  A  field  in  which  colloid  chemistry  finds  many 
applications  is  that  of  food  chemistry.  We  see  an  illus- 
tration of  this  in  the  refining  of  sugar.  In  this  the  sugar 
is  separated  from  its  colloid  accompaniments  by  processes 
of  diffusion,  of  dialysis,  etc.  We  are  face  to  face  here 
with  technical  questions  through  the  solution  of  which  I 
was  told  in  America  fortunes  may  be  made.  Various  cane 
sugar  molasses  containing  great  quantities  of  sugar  are 

1  See  especially  the  reviews  of  F.  GOLDSCHMIDT,  Roll  -Zeitschr.,  2,  193, 
227,  287  (1908);   5,  81  (1909);   8,  39  (1910);    J.  LEIMDORFER,  Kolloidchem. 
Beih.,  2,  343  (1911).     Numerous  other  papers  and  references  to  such  appear 
in  the  Kolloid-Zeitschrift. 

2  See  especially  W.  SPRING,  Koll.-Zeitschr"  4,  161  (1910);  6,  11,  101,  164 
(1911). 


TECHNICAL  APPLICATIONS  215 

sold,  for  instance,  as  cattle  feed,  simply  because  the  sugar 
cannot  be  separated  from  its  colloid  accompaniments.  We 
no  doubt  deal  in  these  instances  with  adsorption  com- 
pounds between  pectin-like  substances  and  sugar,  and  the 
colloid-chemical  problem  involved  is  that  of  the  destruc- 
tion of  this  combination. 

In  the  official  testing  of  foodstuffs,  colloid-chemical 
methods  are  used  for  the  discovery  of  adulterants.  I  have 
already  touched  upon  LEY'S  silver  test  for  the  distinction 
of  natural  from  artificial  honey.  A  colloid-chemical  method 
for  discovering  the  addition  of  agar  to  fruit  jellies  and 
marmalades  makes  use  of  the  influence  which  such  addi- 
tion has  upon  the  form  and  the  structure  of  LIESEGANG 
rings  when  formed  in  such  jellies.1 

But  the  preparation  of  our  foods  is  in  large  measure  a 
colloid-chemical  one.  Bread-making  consists  in  the  forma- 
tion of  a  starch  and  protein  gel  possessed  of  a  definite 
structure.  When  bread  gets  stale  a  change  has  occurred 
in  the  internal  state  of  this  gel.  There  has  occurred 
a  decrease  in  the  degree  of  dispersion,  a  dehydration 
and  syneresis.2  The  syneresis  of  ageing  bread  is  clearly 
apparent  when  bread  is  protected  against  evaporation  in 
a  tin  box.  The  bread  assumes,  after  a  day  or  two,  a  moist 
feel,  even  though  no  water  has  come  in  contact  with  it 
from  without.  The  moisture  comes,  as  a  matter  of  fact, 
from  the  body  of  the  bread  itself.  The  stale  bread  is 
unable  to  hold  as  much  water  as  before  and  through  syn- 
eresis the  water  is  brought  to  the  surface.  By  heating  the 
bread  in  the  oven  for  a  tune  this  water  may  be  made  to 
go  back  into  the  bread  —  just  as  when  a  syneretic  gelatin 
gel  is  warmed  slightly  —  and  in  this  fashion  the  earlier 
state  of  the  bread  is  in  part  restored.  Syneresis  is,  as  a 

1  See  the  paper  of  E.  MARRIAGE,  Koll.-Zeitschr.,  11,  1  (1912). 

2  See  also  the  newer  investigations  of  this  problem  by  J.  R.  KATZ,  Zeitschr. 
f.  Elektrochemie  (1912).    His  general  concepts  are  somewhat  more  compli- 
cated than  mine,  while  that  of  syneresis  receives  no  consideration  at  all. 
A  detailed  monograph  by  KATZ  is  shortly  to  appear  from  the  press  of  TH. 
STEINKOPFF  in  Dresden. 


216  COLLOID  CHEMISTRY 

matter  of  fact,  a  phenomenon  very  common  in  our  foods. 
The  completely  homogeneous  gel  of  freshly  soured  milk 
soon  squeezes  off  enormous  amounts  of  serum  —  the  whey; 
a  freshly  broiled  steak,  in  spite  of  careful  coagulation  of 
its  superficial  layers,  expresses  a  part  of  its  juice. 

The  utilization  of  another  colloid-chemical  principle  is 
seen  in  the  employment  of  gelatin  in  the  manufacture  of 
ice  cream.  It  serves  as  a  protective  colloid  for  the  fine 
crystals  of  ice  which  are  formed  in  the  process  of  freezing, 
giving  these  ice  crystals  a  high  degree  of  dispersion  and 
thus  the  finished  product  its  wished-for  body  and  smooth- 
ness.1 

Colloid-chemical  processes  are  also  encountered  in  the 
production  of  mayonnaise 2  and  of  sauces  of  various  kinds. 
Adsorption  phenomena  are  called  into  service  when  the 
salt  content  of  a  bouillon  is  decreased  so  much  that  the 
effect  may  be  recognized  even  by  the  sense  of  taste,  simply 
by  adding  unsalted  rice  to  the  bouillon.  It  is  also  seen 
when  the  good  housewife  reduces  the  salt  content  of  a  too 
salty  soup  by  stirring  into  it  an  egg.  The  art  of  cooking 
is  a  colloid-chemical  art. 

But  coffee  and  tea  are  also  colloid  solutions.  The 
former  lends  itself  splendidly  to  demonstration  experi- 
ments in  diffusion,  dialysis,  electrophoresis,  ultramicros- 
copy,  coagulation,  adsorption,  etc.  Wine  and  beer,  too, 
are  colloid  solutions.3  The  colloids  of  beer  are  positively 
charged  and  their  presence  gives  beer  its  body  and  its 
foaming  qualities.  A  beer  problem  of  a  distinctly  colloid- 
chemical  nature  has  proved  of  interest  to  Americans. 
Since  they  are  accustomed  to  consume  their  beer  at  a 
temperature  lying  near  the  freezing  point,  the  beer  was 
found  often  to  become  turbid.  This  is  due  to  the  fact 

1  See  J.  ALEXANDER,  Koll.-Zeitschr.,  6,  101  (1909);  6,  197  (1910). 

2  See  MARTIN  H.  FISCHER  and  MARIAN  O.  HOOKER,  Fats  and  Fatty 
Degeneration,  New  York,  1917. 

3  The  colloid  chemistry  of  beer  is  discussed  in  the  numerous  papers  of 
F.  EMSLANDER  appearing  in  the  Kolloid-Zeitschrift.    See  also  the  summary 
of  P.  EHRENBERG,  Koll.-Zeitschr.,  4,  76_(1909). 


TECHNICAL  APPLICATIONS  217 

that  the  colloid  proteins  of  beer  tend  to  precipitate  at  these 
low  temperatures.  By  adding  hydrating  substances  to  the 
beer  like  lactic  acid  or  traces  of  proteolytic  ferments,  it 
was  found  possible  to  hydrate  the  proteins  so  heavily  that 
they  no  longer  settled  out.  This  trick  must  impress  every 
colloid  chemist  as  highly  amusing,  for  to  accomplish  it 
the  very  substances  are  used  which  in  the  body  are  respon- 
sible for  the  production  of  edema.  The  Americans  liter- 
ally consume  beer  which  has  been  rendered  "edematous." 

I  must  add  that  in  the  tar  industry,  in  the  crude  oil 
industry  and  in  many  others  there  appear  countless  dis- 
persed systems  in  the  form  of  emulsions,  of  fogs,  etc., 
the  prevention  or  destruction  of  which  constitutes  the  most 
difficult  and  important  of  the  various  problems  encoun- 
tered in  these  industries.  In  the  emulsification  and  stabil- 
ization of  the  resulting  emulsion  as  observed  in  oleomar- 
garin  manufacture,  we  also  encounter  problems  and 
processes  of  colloid-chemical  interest.  The  number  of 
technical  processes  in  which  comminution,  suspension, 
clarification,  diffusion,  filtration  and  other  processes  are 
used  in  order  to  obtain  various  ends,  and  the  relationships 
of  these  to  colloid  chemistry  is,  as  a  matter  of  fact,  so  large 
and  so  apparent  that  mere  reference  to  them  is  sufficient. 
Dispersoid-chemical  processes  are  used  to  overcome  dust 
and  smoke.  COTTRELL'S  method  to  this  end  makes  use 
of  the  well-known  phenomenon  of  electrophoresis  hi  dis- 
persed particles. 

Finally,  when  I  tell  you  that  a  large  part  of  the  refuse 
materials  found  in  our  drainage  systems  (constituting  in 
city  sewers,  for  example,  fifty  to  sixty  percent  of  the  solids 
contained  in  the  waters  flowing  through  them)  is  found 
here  in  a  colloid  state  and  that  colloid-chemical  methods 
must  in  consequence  be  used  to  handle  them,  you  will 
perhaps  be  inclined  to  agree  with  me  when  I  say  that 
things  not  only  begin  in  colloid  chemistry,  but  in  colloid 
chemistry  they  end. 


218  COLLOID  CHEMISTRY 

§18. 

With  this  I,  too,  must  conclude  not  only  my  remarks  upon 
the  technical  applications  of  colloid  chemistry  but  the  entire 
series  of  my  lectures.  I  shall  be  satisfied  if  I  have  suc- 
ceeded in  making  clear  to  you  the  newness  of  colloid  chemis- 
try, its  wealth,  and  its  inexhaustible  possibilities  of  scientific 
and  practical  application.  It  is  these  which  I  think  justify 
us  in  looking  upon  colloid  chemistry  as  entitled  to  independ- 
ent existence. 

In  retrospect  you  will,  perhaps,  be  tempted  to  ask  me  the 
following  questions.  If  it  is  true  that  we  are  dealing  with 
a  science  so  rich  in  ideas  and  so  pregnant  with  possibilities 
of  scientific  application  —  I  presume  too  much,  perhaps, 
when  I  assume  that  my  lectures  have  given  you  this  im- 
pression —  if  all  this  is  true,  why  is  it  that  colloid  chemistry 
has  not  long  been  known  as  an  independent  science?  Why 
is  it  that  this  science  which  deals  in  such  large  measure  with 
commonplace  and  everyday  sorts  of  things  has  been  studied 
systematically  for  but  a  few  years? 

I  think  that  the  answers  to  these  questions  are  about  as 
follows.  Physics  has  until  recently  busied  itself  chiefly  with 
the  properties  of  matter  in  mass;  chemistry,  on  the  other 
hand,  has  dealt  chiefly  with  the  smallest  particles  of  matter 
such  as  atoms  and  molecules.  Relatively  speaking,  we 
know  much  of  the  properties  of  large  masses  and  we  talk 
much,  also,  of  the  properties  of  molecules  and  atoms.  It 
is  because  of  this  that  we  have  been  led  to  regard  everything 
about  us  either  from  the  standpoint  of  physical  theory  or 
from  that  of  molecular  or  atomic  theory.  We  have  entirely 
overlooked  the  fact  that  between  matter  in  mass  and  matter 
in  molecular  form  there  exists  a  realm  in  which  a  whole 
world  of  remarkable  phenomena  occur,  governed  neither  by 
the  laws  controlling  the  behavior  of  matter  in  mass  nor  yet 
those  which  govern  materials  possessed  of  molecular  dimen- 
sions. We  did  not  know  that  this  middle  country  existed, 
how  large  a  number  of  natural  phenomena  belonged  in  it, 


TECHNICAL  APPLICATIONS  219 

nor  how  greatly  the  degree  of  dispersion  determined  their 
behavior.  We  have  only  recently  come  to  learn  that  every 
structure  assumes  special  properties  and  a  special  behavior 
when  its  particles  are  so  small  that  they  can  no  longer  be 
recognized  microscopically  while  they  are  still  too  large  to 
be  called  molecules.  Only  now  has  the  true  significance  of 
this  region  of  the  colloid  dimensions -- THE  WORLD  OF 
NEGLECTED  DIMENSIONS  —  become  manifest  to  us. 

If  some  of  my  explanations  have  seemed  not  clear  or 
inadequate,  I  beg  you  to  consider  this  not  as  characteristic 
of  colloid  chemistry,  but  as  dependent  solely  upon  my 
personal  shortcomings.  A  science  may  attain  to  fullness; 
her  disciples,  never. 


INDEX 


AUTHOR  INDEX 


A. 

ACHESON,  182,  183. 
ALEXANDER,  J.,  216. 
AMANN,  J.,  18. 
AMBRONN,  H.,  194. 
ARRHENIUS,  SVANTE,  144. 
ATTERBERG,  A.,  152. 
AVOGADRO,  141,  142. 

B. 

BACHMANN,  91. 

BARKLA,  A.,  54. 

BECHHOLD,  H.,  47,  48,  74. 

BELTZER,  J.  G.,  209. 

BENEDICKS,  C.,  34,  198,  199,  202,  203. 

BERMAN,  46,  72,  73,  135. 

BERNSTEIN,  G.,  212. 

BEYERINCK,  N.,  158. 

BILTZ,  W.,  124. 

BIRCHER,  E.,  174. 

BOSE,  M.,  139. 

BOTTAZZI,  F.,  51,  155,  156,  157. 

BREDIG,  G.,  33,  75. 

BREHM,  H.,  152. 

BROWN,  44,  45,  47,  57,  59,  142,  157. 

BUTSCHLI,  O.,  94,  100,  102,  140. 

BYSOW,  B.,  210. 

C. 

CAVAZZI,  A.,  195. 
CHIARI,  R.,  161. 
CHRISTIANSEN,  6,  58. 
COMTE,  A.,  127. 
CORNU,  F.,  147,  151. 

COTTRELL,  217. 

D. 

DlTMAR,  D.,  210. 

DOHLE,  W.,  85. 
DONAU,  J.,  130,  131. 


E. 

EGGERTZ,  200,  201. 
EHRENBERG,  P.,  131,  152,  216. 
EMSLANDER,  F.,  216. 

F. 

FARADAY,  M.,  13,  22,  49,  71,  83. 
FIGHTER,  F.,  69. 
FICK,  A.,  156. 
FISCHER,  A.,  169. 
FISCHER,  MARTIN  H.,  46,  72,  73,  135, 

155,  160,  164,  166,  171,  172,  173, 

174,  216. 
FREUNDLICH,  H.,  70,  83. 

G. 

GAIDUKOW,  N.,  101,  155. 
GIBBS,  WILLARD,  81,  122,  198. 
GIES,  W.,  171. 

GOLDSCHMIDT,  F.,  214. 

GODLEWSKI,  T.,  141. 

GRAHAM,  3,  5,  7,  8,  9,  11,  66,  82,  92, 

93. 
GURTLER,  W.,  197,  199,  200,  202,  203, 

204. 
GURWITSCH,  L.,  117. 

H. 

HANDOVSKY,  H.,  112. 
HARDY,  W.,  94. 

HATSCHEK,  E.,  86,  104,  105,  106,  107. 
HAUSER,  O.,  133. 
HEYN,  203. 

HINRICHSEN,  F.  W.,  210. 
HOBER,  R.,  155. 
HOFMEISTER,  F.,  113,  156,  162. 
HOOKER,  MARIAN  O.,  216.    . 


1. 


ISHIZAKA,  N.,  83. 


223 


224 


AUTHOR  INDEX 


K. 

KANGRO,  WM  54. 
KAUFFMANN,  M.,  175. 
KARCZAG,  L.,  6. 
KATZ,  J.  R.,  215. 
KEISERMANN,  S.,  192. 
KITE,  G.  L.,  163. 

KOHLSCHUTTER,  V.,   196. 

KOSSONOGOW,  J.  J.,  71. 
KUSTER,  E.,  106,  150. 
KUZEL,  185. 

L. 

LANGE,  O.,  188. 
LAUE,  M.,  54. 
LEA,  Carey,  11,  33,  62. 
LEIMDORFER,  J.,  214. 
LEY,  131,  215. 
LEYDE,  203. 
LIEBIG,  170. 
LIESEGANG,  R.  E.,  61,  105,  106,  107, 

108,  149,  189,  215. 
LINDER,  S.  E.,  47. 
LLOYD,  J.  U.,  119. 
LOEB,  J.,  163. 
LOEWE,  S.,  175. 
LORENZ,  R.,  204. 
LOTTERMOSER,  A.,  70,  115,  142. 
LUDWIG,  C.,  49,  156. 
Ltfppo-CRAMER,  61,  62,  132,  189. 

M. 

MALFITANO,  G.,  10,  47. 
MANN,  G.,  169. 
MARRIAGE,  E.,  215. 
MARTIN,  C.  J.,  47. 
MAYER,  119. 
MECKLENBERG,  W.,  133. 
MENZ,  W.,  91. 

MICHAELIS,  W.,  148,  192,  193. 
MILLER,  E.,  196. 
MULLER,  E.,  34. 

N. 

NAUMANN,  188. 
NAVASSART,  E.,  66. 
NEUBERT,  J.  K.,  190,  191. 
NEUNER,  CHR.,  205. 


O. 

ODEN,  S.,  41,  116,  152. 
OESPER,  46,  72,  73,  135. 
OHLON,  E.,  116. 

OSTWALD,  WlLHELM,   107,   127. 

OSTWALD,  WOLFGANG,  34,  41,  46,  53, 
62,  67,  71,  72,  73,  91,  94,  96,  97, 133, 
146,  210. 

OSWALD,  A.,  174. 

P. 

PAAL,  5,  6. 
PAINE,  H.,  83. 
PANETH,  F.,  141. 
PAULI,  WOLFGANG,  112, 113,  156, 161, 

164,  166,  204. 
PAWLOW,  P.,  74. 
PELET-JOLIVET,  J.,  205. 
PERNTER,  J.  M.,  143. 
PERRIN,  J.,  74,  142. 
PICTON,  H.,  47. 
PIERONI,  A.,  75. 
POPPER,  186. 

Q. 
QUINCKE,  G.,  64,  69,  100. 

R. 

RAFFO,  M.,  75. 
RAMSAY,  W.,  47. 
RAYLEIGH,  LORD,  59. 
REINDERS,  W.,  132. 
RHUMBLER,  L.,  154,  159. 
RICHTER,  BENJ.  JEREMIAS,  3,  22. 
ROHLAND,  P.,  192. 

RONTGEN,   16,  54. 
RUHLAND,  W.,   167. 

RUSZNYAK,  ST.,  75. 

S. 

SABATIER,  138. 
SAHLBOM,'  N.,  69. 
SAMEC,  M.,  96. 
SANIN,  A.,  124. 
SCHAPER,  A.,  165. 
SCHIDROWITZ,  P.,  210. 
SCHOEP,  48. 
SCHWARZSCHILD,  K.,  144. 


AUTHOR  INDEX                                        225 

SELMI,  F.,  3.  VON  BACHMETJEW,  74. 

SIEDENTOPF,  H.,  55,  135,  145.  VON  HEYDEN,  185. 

SPENCE,  D.,  210,  211,  212.  VON  JUPTNER,  H.,  201. 

SPRING,  W.,  52,  54,  56,  214.  VON  SCHROEDER,  J.,  206. 

STAS,  G.,  72.  VON  WEIMARN,  P.  P.,  21,  25,  26,  47, 

STIASNY,  E.,  205.  49,  198. 
STIEGLITZ,  J.,  33. 

STRIETMANN,  W.  H.,  166.  W- 

SVEDBERG,  THE,  6,  34,  47,  60,  61,  63,  WEBER,  C.  O.,  51. 

64,  65,  67.  WEGELIN,  G.,  34. 

rp  WENZEL,  75. 

WlNTERSTEIN,  51. 

THOMSON,  J.  J.,  122.  WOHLER,  170. 

TYNDALL,  J.,  50,  51,  52,  53,  54,  55,  56,  W5HLER  L    65 
58,  136. 

V.  Z. 

VAN  BEMMELEN,  J.  M.,  124,  154.  ZIRKEL,  188. 

VAN'T  HOFF,  42,  136,  198.  ZSIGMONDY,  R.,  13, 55, 56,  57, 91, 102. 


SUBJECT  INDEX 


A. 

ACACIA,  97. 

Acetyl  cellulose  ester,  209. 

Acheson  graphite,  183. 

Acids  and  swelling,  98;  and  edema, 
171;  and  cement,  194. 

Adulterants  in  honey,  131,  215. 

Adsorption,  81,  116;  and  chemical 
consequences,  117,  154;  of  alka- 
loids, 119;  and  Gibbs'  theorem, 
122;  mutual,  123;  in  soil,  154;  in 
dyeing  and  tanning,  205  to  207;  in 
vulcanization,  212. 

Adsorption  isotherm,  120. 

Agar,  92. 

Albumin  (see  also,  Protein),  82,  92. 

Alkali  blue,  68. 

Alkalies  and  swelling,  98;  and  liquid 
ammonia,  133;  and  clays,  189. 

Alkaloids,  119. 

Allotrophy,  200. 

Alloys,  197  to  205. 

Aluminium  silicate,  190. 

Ammonium  sulphate,  82. 

Analysis,  colloid,  7. 

Aniline  dyes,  60. 

Animal  membranes,  9. 

Anthrapurpurin,  192. 

Antipyrin,  74. 

Applications  of  colloid  chemistry  to 
analytical  chemistry,  129;  to  or- 
ganic chemistry,  133;  to  dyeing, 
135,  205  to  207;  to  physical  chem- 
istry, 136;  to  catalysis,  138;  to 
crystalline  liquids,  139;  to  radio- 
activity, 141;  to  Avogadro's  con- 
stant, 141;  to  cosmic  physics,  144; 
to  mineralogy,  145  to  150;  to 
geology,  151;  to  soil  chemistry, 
151  to  155;  to  biology,  155  to  165; 
to  morphology,  164;  to  growth, 
165;  to  muscular  contraction,  166; 


to  secretion,  167;  to  vital  staining, 
168;  to  synthetic  biochemistry, 
170;  to  pathology  and  medicine, 
171  to  175;  to  technology,  179  to 
219;  to  lubricants,  182;  to  light 
filaments,  184;  to  glass,  186;  to 
ultramarine,  187;  to  pigments, 
188;  to  photography,  189;  to 
ceramics,  189;  to  hydraulic  ce- 
ments, 191  to  195;  to  metallurgy, 
195;  to  alloys,  197  to  199;  to  steel, 
200  to  205;  to  dyeing  and  tanning, 
135,  205  to  207;  to  cellulose  in* 
dustries,  208;  to  silk  manufacture, 
209;  to  rubber,  210;  to  vulcaniza- 
tion, 211  to  213;  to  soap  manu- 
facture, 214;  to  food  chemistry, 
214  to  217. 

Aquadag,  182. 

Artificial  silk,  208. 

Arsenic  trisulphid,  47. 

Astrospheres,  164. 

Austenite,  201. 

Avogadro's  constant,  141. 

B. 

BAKELITE,  209. 
Barium  sulphate,  26. 
Beer,  216,  217. 
Benzol,  114. 
Berlin  blue,  24. 

Biology,  155  to  170;  synthetic,  170. 
Blue  rock  salt,  145. 
Borax  beads,  130. 
Bread,  215. 
Bricks,  181,  183. 
Brownian  movement,  44,  45,  142. 

C. 

CALCIUM  CARBONATE,  106. 
Calcium  sulphate,  195. 
Capillary  analysis,  68. 


227 


228 


SUBJECT  INDEX 


Capillary  phenomena,  73. 

Carbon,  117,  118,  200,  203,  204. 

Cassius  purple,  129. 

Catalyst,  137,  138,  139. 

Catalytic  effects,  75. 

Cellon,  209. 

Cellulose,  208. 

Cement,  191  to  195. 

Cementite,  200. 

Ceramics,  189. 

Chamberland  filter,  47. 

Changes  in  state,  87. 

Christiansen  colors,  6,  58. 

Chromisomers,  133. 

Chromium,  186. 

Cinnamic  ethyl  ester,  58. 

Classification  of  colloids,  39,  40,  41, 
42. 

Clays,  189,  190. 

Clay  sphere,  73. 

Clothes,  179. 

Clouds,  144. 

Coagulation,  80,  83;  of  colloids,  110, 
111,  112,  113,  114. 

Coffee,  216. 

Colloids,  definition  of,  4,  34,  76;  con- 
cept of,  5,  218,  219;  examples  of, 
5;  analysis  of,  7;  diffusion  of,  7; 
membranes  of,  8;  and  relation  to 
suspensions,  9,  13,  19,  20,  34;  and 
relation  to  molecular  solutions,  13, 
19,  20,  34;  preparation  of,  22,  26, 
33;  of  gold,  22;  classification  of, 
39,  42;  properties  of,  44;  Brownian 
movement  in,  44;  diffusion  and 
dialysis  of,  45;  salts  and  hydrated, 
46;  filtration  of,  48;  optical 
properties  of,  49;  Tyndall  cone  in, 
50;  hydrated,  51;  ultramicroscopy 
of,  56;  of  sodium  chlorid,  58;  and 
maximum  properties,  59;  and 
colors  of,  58,  59,  60;  and  law  of 
color  in,  61,  62;  and  transition 
phenomena,  63;  and  electrical  be- 
havior, 67;  and  velocity  of  migra- 
tion, 70;  and  freezing  and  melting 
points,  72,  73,  74;  and  capillarity, 
73;  and  reaction  velocity,  75;  in- 


ternal changes  in,  79;  adsorption 
in,  81;  instability  of,  82;  and  kinet- 
ic treatment,  82;  viscosity  of,  83; 
and  temperature,  85;  and  time 
factor,  86;  and  added  substances, 
86;  and  critical  mixtures,  88;  and 
foaming,  90;  and  separation  phe- 
nomena in,  91;  and  syneresis,  93, 
100;  and  swelling,  95,  96,  97;  ef- 
fect of  additions  to,  98;  pressures 
and  swelling  of,  99;  and  structure, 
100,  101;  as  gels,  102;  periodic 
precipitations  in,  104  to  107;  frost 
figures  in,  108;  coagulation  of,  110, 
111,  112,  113,  114;  protective,  113; 
and  radiant  energy,  114;  and  pep- 
tization,  115;  and  adsorption,  116 
to  124;  and  Gibbs'  theorem,  122; 
importance  of,  127;  of  noble 
metals,  129,  130,  131;  and  honey, 
131;  and  analytical  chemistry,  129; 
and  organic  chemistry,  133;  and 
dyeing,  135,  205,  206,  207;  and 
physical  chemistry,  136;  and  catal- 
ysis, 138;  and  crystalline  liquids, 
139;  and  radioactivity,  141;  and 
Avogadro's  constant,  141;  and 
cosmic  physics,  144;  and  miner- 
alogy, 145  to  150;  and  geology, 
151;  and  soil  chemistry,  151  to  155; 
and  biology,  155  to  170;  and  medi- 
cine, 171  to  175;  and  technology, 
179  to  219;  and  lubricants,  182; 
and  incandescence,  184;  and  glass, 
186;  and  ultramarine,  187;  and 
pigments,  188;  and  photography, 
189;  and  ceramics,  189;  and  hy- 
draulic cements,  191  to  195;  and 
metallurgy,  195;  and  alloys,  197 
to  199;  and  steel,  200  to  205;  and 
dyeing  and  tanning,  135,  205,  206, 
207;  and  cellulose  industries,  208; 
and  silk  manufacture,  209;  and 
rubber  manufacture,  210  to  213; 
and  soap  manufacture,  214;  and 
food  chemistry,  214  to  217. 

Colloid  chemistry,  3;  history  of,  4. 

Colloid  ions,  71. 


SUBJECT  INDEX 


229 


Colloid  silver,  33,  131. 

Colloid  state,  21,  35. 

Collodion  capsules,  10,  48. 

Color  of  colloids,  58. 

Colors,  Christiansen,  6,  58;  intensity 
of,  60;  of  gold,  61;  law  governing, 
62;  of  metals,  64;  of  indigo,  65; 
of  clothes,  179. 

Concentration  and  degree  of  disper- 
sion, 25. 

Concentration  function,  120. 

Condensation  method,  22,  25. 

Conductivity  in  colloids,  70;  in 
gases,  71;  in  electrolytes,  71. 

Constant,  Avogadro's,  141. 

Contraction,  muscle,  166. 

Cooking,  216. 

Cosmic  physics,  143,  144,  145. 

Cotton,  208. 

Critical  mixtures,  88. 

Critical  temperature,  88. 

Crystalline  forces,  46. 

Crystalline  liquids,  139,  140. 

Crystalloids,  diffusion  of,  8. 

Crystals,  swelling  of,  95;  absorption 
of  dyes  by,  132,  133. 

D. 

DAYLIGHT,  143. 
Decimal  division,  118. 
Degree  of  dispersion,  17,  18,  43;    in 

Berlin    blue,    24;     and    diffusion, 

46. 

Dialysis,  9,  44. 
Dialysers,  9. 
Diffraction,  50,  55. 
Diffusion,  7,  44,  45,  47. 
Diffusion  coefficient,  47. 
Dispersed  systems,  15,  16,  17,  18,  20, 

42;   properties  of,  43;  mechanical 

properties  of,  43. 
Dispersoids,  15,  16,  17,  18,  20. 
Dispersion  degree,  43. 
Dispersion  method,  22. 
Drainage,  217. 
Dust,  cosmic,  144. 
Dyeing,  205  to  207;  fast,  207. 
Dyes,  135. 


E. 

EDEMA,  171,  172,  173. 

Egg  white,  114. 

Electrical  behavior  of  colloids,  67. 

Electrical  dispersion,  33. 

Electrolytes  and  colloids,  86,  112; 
and  swelling,  98;  and  peptization, 
115,  116. 

Electrolytic  metallurgy,  196. 

Emulsification,  217. 

Emulsoids,  40,  80,  85,  89,  90;  coagu- 
lation of,  112. 

F. 

FARADAY'S  LAW,  71. 
Fast  dyeing,  207. 
Ferrite,  201. 
Ferrocyanid,  24. 
Fertilization,  163. 
Fibrin,    98;    and  water   absorption, 

161. 
Filter  paper,  47;  and  colloid  analysis, 

68;  freezing  of,  73,  74. 
Filters,  11,  19,  47. 
Flea  bites,  172. 
Foams,  41,  90. 
Fog,  42,  217. 

Food  chemistry  and  colloids,  214. 
Freezing  point,  72,  73,  74. 
Frog,  developing,  165. 
Frog  eggs,  159. 
Frost  figures,  108. 
Fuller's  earth,  114,  117. 

G. 

GALALITE,  209. 

Gas  -f  liquid  dispersoids,  41,  42. 

Gas  +  solid  dispersoids,  42. 

Gelatin  and  color  of  colloids  in,  61; 
and  viscosity,  84,  85,  86,  92;  syn- 
eresis  in,  93;  swelling  of,  98; 
precipitations  in,  104;  and  glass- 
chipping,  111;  and  water  absorp- 
tion, 161;  and  artificial  silk,  209. 

Gels,  80,  93,  100;  freezing  of,  108. 

Geology,  151. 

Gibbs'  rule,  81;  in  alloys,  198. 

Glanzgalvanisation,  196. 


230 


SUBJECT  INDEX 


Glass,  chipping  of,  111;    ruby,  186; 

blue,  186. 
Glaucoma,  174. 
Glue,  5,  94;  in  cement,  194. 
Gold  chlorid,  22; 
Gold,  colloid,  22,  61,  62,  68;    colors 

of,  61,  62;    freezing  of,  73;    tests 

for,  129;  in  glass,  186. 
Graphite,  69,  182. 
Gravity  and  dispersoids,  142. 
Gredag,  184. 
Growth,  165. 
Gum  arabic,  80. 

H. 

HETEROGENEOUS  SYSTEMS,  16. 

Hides,  205. 

Homogeneous  systems,  16. 

Honey,  131. 

Humus  acids,  131,  152. 

Hyaline  minerals,  147. 

Hydrated  colloids,  51,  84;  coagula- 
tion of,  112. 

Hydrates,  133. 

Hydration,  51,  133;  of  protoplasm, 
159,  160. 

Hydraulic  cements,  191  to  195. 

I. 

INCANDESCENT  FILAMENTS,  184. 
Ice  cream,  216. 
Ice  crystals,  108. 
Indigo,  65. 
Inflammation,  174. 
Infusorial  earth,  39. 
Ink,  180. 

Instability  of  colloids,  82. 
Intensity  of  colors,  60. 
Ions,  70;   coagulating,  111;   stabiliz- 
ing, 115,  116. 

Iron  alloys,  200,  201,  202,  203. 
Iron  ferrocyanid,  24. 
Iron  hydroxid,  68. 
Isobutyric  acid- water,  91. 
Isochemites,  147. 
Isocolloids,  134,  213. 
Isopren,  134. 


K. 
KOLLOID-ZEITSCHRIFT,  181.< 

L. 
LAW,  von  Weimarn's,  25;  Ostwald's, 

61,  62;    Faraday's,  71;    Wenzel's, 

75;  Gibbs',  81. 
Lead  chromate,  104,  106. 
Leather,  179. 
Ley's  test,  131,  215. 
Liesegang  rings,  104,  149,  215. 
Life  processes,  155. 
Light  pressure,  144. 
Liquid  +  gas  dispersoids,  42. 
Liquid  +  liquid  dispersoids,  42,  80. 
Liquid  +  solid  dispersoids,  41. 
Living  matter,  155  to  158. 
Lloyd's  reagent,  119,  120. 
Localization,  162. 

M. 

MARTENSITE,  201. 

Maximum  and  colloid  realm,  59,  75; 

absorption,  62. 
Mayonnaise,  216. 
Mechanical  properties,  43. 
Melting  point,  72. 
Membrane,  9. 
Mercerized  cotton,  208. 
Mercuric  sulphid,  11. 
Metals,  dispersion  of,  33. 
Metallurgy,  195. 
Micro-dissection,  163. 
Mineralogy,  145. 
Mining,  195. 
Molasses,  214. 

Molecular  solutions,  13,  18,  19. 
Molecules,  11. 
Mortar,  191. 
Muscular  contraction,  166. 

N. 

N,  141,  142. 

Neglected  dimensions,  219. 
Night  blue,  68. 

Noble  metals,  recognition  of,  129. 
Noble  rubber,  213. 


SUBJECT  INDEX 


231 


Non-electrolytes    and    colloids,    86; 
and  swelling,  98. 

O. 

OCHRES,  188. 
Oildag,  182. 
Opal,  148,  149. 
Opalescence,  57,  89. 
Optical  properties,  49. 
Optical  rotation,  66. 
Order  of  color  change,  62. 
Organic  chemistry  and  colloids,  133. 
Osmondite,  201. 

Osmotic  pressure  and  water  absorp- 
tion, 160. 

P. 

PAINTS,  188. 

Parchment  paper,  9,  208. 

Parthenogenesis,  164. 

Paste  precipitates,  26. 

Pathology,  171. 

Peptization,  80,  115;  in  clays,  190. 

Periodicity,  14,  17. 

Perlite,  200,  201. 

Petroleum,  114. 

Phases,  92. 

Phenol,  209. 

Phenol-water,  88,  90. 

Photographic  plates,  61. 

Photography,  54,  132,  189. 

Physical  chemistry  and  colloids,  136. 

Pig  bladder,  9,  49. 

Pigments,  188. 

Plaster-of-Paris,  195. 

Plastic  masses,  208. 

Platinum,  colors  of,  64;  and  hydro- 
gen peroxid,  75;  tests  for,  130. 

Points,  82. 

Pores,  19,  47. 

Potassium,  swelling  of,  95. 

Precious  stones,  146. 

Precipitates,  26,  27,  28,  29,  30,  31,  32; 
periodic,  104;  and  analytical  chem- 
istry, 129. 

Pressure  of  swelling,  99. 

Protective  colloids,  113. 

Protein,  82,  92,  98. 


Protoplasm,  155,  156>  157,  158. 
Pyrosols,  204. 

Q. 

QUARRYING,  180. 
Quartz,  solubility  of,  148. 

R. 

RADIO-CHEMISTRY,  128,  141. 

Reaction  velocity,  75. 

Reagent,  Lloyd's,  119,  120;  Mayer's, 
119. 

Red  gold,  22,  23,  186. 

Red  ochre,  188. 

Refraction,  49,  50. 

Reichel  filter,  47. 

Reversibility,  116. 

Rings,  Liesegang,  104  to  107, 149,  150. 

Rock  salt,  blue,  144. 

Rontgen  rays,  16,  54. 

Rotation,  optical,  66. 

Rubber,  swelling  of,  95;  in  manu- 
facture, 210;  vulcanization  of,  211; 
synthesis  of,  134,  212. 

Rubies,  186. 

Ruby  glass,  186. 

S. 

SALOL,  74. 

Salve-like,  85. 

Secretion,  167. 

Selenium,  186. 

Semipermeable  membranes,  48. 

Separation  in  critical  fluids,  91;  into 
phases,  93. 

S:rum,  94. 

Silicic  acid,  93;  minerals  of,  147. 

Silk,  artificial,  208. 

Silver  chromate,  104,  105. 

Silver  iodid,  peptization  of,  115. 

Silver,  11;  dispersion  of,  33;  haloids 
of,  61;  colors  of,  61,  64,  132;  solu- 
tions of,  133;  colloids  of,  175;  in 
glass,  186. 

Sky,  143,  144. 

Smoke,  42. 

Soap,  214. 

Sodium,  swelling  of,  95. 


232 


SUBJECT  INDEX 


Sodium  eWorld,  6,  58. 

Sodium  sulphate,  49. 

Soil  chemistry,  151  to  155. 

Solid  solution,  198. 

Solid  +  gas  dispereoids,  42. 

Solid  +  liquid  dispersoids,  40,  41,  80. 

Solid  +  solid  dispersoids,  42. 

Sols,  80. 

Solubility,  72. 

Solution,  10;  of  alkali  metals,  133. 

Solvation,  51,  87,  134,  136. 

Sorbite,  201. 

Specific  surface,  73. 

Stabilization,  89,  92. 

Staining,  168. 

Starch,  95,  96. 

Steel,  200  to  205. 

Straw,  183. 

Structure,  100,  102,  169. 

Sugar,  214. 

Sulphur,  11,  18,  41,  85,  187;  in  rub- 
ber vulcanization,  211. 

Sulphur  dyestuffs,  188. 

Surface,  specific,  73;  and  absorption, 
117,  118. 

Suspension  colloids,  40,  80. 

Suspensions,  13,  40. 

Suspensoids,  40,  80. 

Swelling,  94;  in  vapor,  97;  effect  of 
added  substances  upon,  98;  and 
syneresis,  100,  101. 

Syneresis,  93,  100,  101;  in  bread, 
215;  in  meats,  216. 

Synthesis  of  rubber,  134,  212. 

Synthetic  biology,  170. 

T. 

TANNIN,  66,  183. 
Tanning,  205  to  207. 
Tantalum,  184. 
Tea,  216. 

Temperature  and  colloids,  85. 
Textiles,  205. 


Therapy  with  inorganic  colloids,  175- 

Time,  86. 

Transition  phenomena,  63. 

Troostite,  203. 

Tungsten,  184. 

Turbidities,  49;  of  molecular  solu- 
tions, 53. 

Tyndall  cone,  50;  and  degree  of  dis- 
persion, 52;  of  Rontgen  rays,  54. 

Types  of  colloids,  40;  of  dispersed 
phases,  80,  103. 

U. 

ULTRAFILTRATION,  48,  54. 
Ultramarine,  187. 
Ultramicrons,  57. 
Ultramicroscope,  55,  56. 
Ultraviolet  rays,  53. 
Umber,  188. 

V. 

VALENCE  AND  COAGULATION,  111. 
Variability  in  electrical  behavior,  69. 
Vectorial  forces,  46. 
Velocity  of  colloid  migration,  70. 
Vital  staining,  168. 
Violet  gold,  22. 
Viscose,  208. 
Viscosity,  83;    of  emulsoids,  84,  85; 

of    suspensoids,     84;      of    critical 

fluids,  90. 
Von  Weimarn  law,  25. 

W. 
WATER   CONTENT   OF   PROTOPLASM, 

159. 

Water-isobutyric  acid,  91 
Water-phenol,  88,  90. 
Wave  lengths  and  color,  62. 
WenzePs  law,  75. 
Wood,  179. 

Y. 
YELLOW  OCHRE,  188. 


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